One-Dollar Retainer

Technically, a retainer is a deposit against future charges made by a lawyer or other professional -- essentially paying in advance for a reasonable amount of services from them. As those services are performed, payment for them is taken from the retainer, which is held in escrow until it is used up (and can be reclaimed by the person who has paid it if he is unsatisfied with the professional's performance). And while the retainer fee remains unexhausted, the professional is not only working for the person who paid it, they also owe him both loyalty (they can't accept work or new clients which conflict with his interests) and confidentiality (they cannot reveal to anyone anything that might adversely affect him). (And the latter continues even after the retainer runs out -- just because a lawyer isn't representing you any more doesn't mean he can idly blab about your business.)

Since retainer fees are payments in advance for (expensive) professional services, they are usually large -- several thousand US dollars is not excessive and possibly on the low side, not when lawyers (for instance) can charge US$20 a minute for their time.

However, it's not uncommon in entertainment media to see a "token" One-Dollar Retainer accepted by a sympathetic or outright friendly professional person in order to deliberately invoke the loyalty and confidentiality aspects of the subsequent relationship (or any other specialized benefits it might have) -- either to reassure the character doing the paying, to take advantage of the benefits of the formalized relationship, and/or to protect both parties against possible legal complications in the future. This is very much a case of Good Lawyers, Good Clients -- the hero encounters a professional whose services he needs who is both skilled and inclined to help him even if he has to essentially do it pro bono. Sometimes the lack of real payment is handwaved away with a statement to the effect of "you can pay me later".

While this may look like Artistic License Law (at least in the case of lawyers), it actually isn't. Although a lawyer or other professional will have a usual schedule of charges for their services, what they actually choose to charge a specific client is completely up to their discretion. So it is entirely plausible for a professional wanting to help someone while technically avoiding calling it "pro bono" work to take a token retainer fee against future charges that will be equally token.

Important! This is not a generic "lawyer on retainer" trope. The fact that the character has or is a lawyer or other professional on retainer isn't enough to qualify the example for this trope. (In fact, it's practically People Sit on Chairs territory.) This trope is strictly about a professional who accepts a discounted -- often vastly discounted, to the point of being purely symbolic -- retainer fee specifically to invoke the ethical considerations explicit in being "hired" by another character, done for dramatic and storytelling purposes -- to secure a reliable ally for a hero, for example. The token fee is even usually the professional's idea -- "give me a dollar, now you've hired me." The professional so engaged does not have to be a lawyer, as long as their services are normally expensive and their loyalty is important in the context of the plot: assassins, bountyhunters and mercenaries, for example, can qualify. As long as they are giving their loyalty and confidence for a symbolic "fee", it is this trope.

Whether they keep their implicit promise afterward... well, that's the stuff from which plot developments are built. But it will be the rare psychopath who will take a token retainer and then renege on it -- if they're inclined to betray the agreement like that, they're far more likely to refuse the retainer and oppose the hero from day one. So expect that anyone, lawyer or not, willing to take a retainer on a task will do what they promised to do.

Contrast with Pro Bono Barter, where the lawyer or other professional's services are "purchased" at what is assumed to be their going rate with equivalent goods or services provided by the client instead of cash.

Has nothing at all to do with discount orthodontia. Or the Old Retainer.

Anime and Manga

 * In episode 24 of El Cazador de la Bruja, as part of a plan to make sure both Ellis and Lirio are protected, Ricardo convinces Nadie to formally hire him to go out and fight L.A., for all the cash she has in her pockets -- which turns out to be a couple of 10-centavo coins (literally one cent, US). A professional bountyhunter, Ricardo gives the assignment all the serious effort and consideration that he would to a job contracted at his normal price.

Fan Works
""If I am your chief employee, what does that make me?" He was silent before saying, "My strong right hand?" I grinned, and he grinned back at me, although he seemed uncertain. "Pay me," I said. "What?" "It's got to be real," I said, "Or it won't work.""
 * Taylor arranging to be Harry's "strong right hand" just before the final confrontation with Voldemort in the Worm/Harry Potter crossover A Wand for Skitter:


 * Harry ends up paying her with a slightly sticky chocolate frog card. Tonks, who witnesses this exchange, correctly identifies it as Harry paying Taylor to murder Voldemort.


 * In chapter 14 of The Past Is Prologue, an NCIS: Los Angeles fanfic by "anonkp", Marty Deeks "hires" his old friend Joe (a former public defender) as his attorney for a one-dollar retainer so he can share some sensitive information and discuss its ramifications with him under attorney-client privilege.
 * Danny Hebert makes a token one-dollar payment to his friend, lawyer Alan Barnes, when seeking Alan's legal advice in chapter 4 of the Worm AU fic The Taste of Peaches by "Grounders10".

Film

 * In the 1994 film version of John Grisham's novel The Client, inexperienced lawyer Reggie Love (played by Susan Sarandon) accepts a one-dollar retainer from main character Mark Sway (Brad Renfro). In a difference from the usual, it's not because she's being helpful or generous, but because she needs the business and he can't afford anyone better.

Literature

 * John Grisham's The Client, as noted in Film above.
 * In The Lawyer's Tale, a 1993 novel by "D. Kincaid" (the pseudonym of infamous Hollywood lawyer Bert Fields), successful Hollywood attorney Harry Cain accepts a yearly one-dollar retainer from Mike Ovitz, the most powerful agent in L.A., so that he'll never be involved in any attempt to sue Ovitz. (This is allegedly a fictionalization of an actual relationship Fields had with a particular agent.)

Live-Action TV

 * In "The Golden Cage", a 1974 episode of the dramatic series Petrocelli, title character Anthony Petrocelli accepts a one-dollar retainer from a Trophy Wife accused of murder after escaping from her rich, abusive husband (and being cut off from his money).
 * In the Freeform series Good Trouble, Jamie Hunter is technically Callie Foster's laywer and their conversations are protected under lawyer-client confidentiality after she puts down a one-dollar retainer in the episode "Imposter".

Real Life
"Many years ago I approached my attorney for a free consultation. When I had told him what was going on he stopped me and asked me if I had a dollar in my wallet. I handed him a dollar. He had his secretary type up a retainer document and then he told me what to do."
 * Surprisingly, this does happen in real life, although there will be paperwork involved that's almost never seen in fiction. In a Quora.com thread about attorney-client privilege, a home improvement contractor recounted his experience:


 * In 1963 Reno, Nevada attorney Harry D. Anderson agreed to defend Thomas Lee Bean for a retainer of $1 in what was at the time a notorious murder case. (Bean was accused, and eventually convicted, of the murder of Olympic skier Sonja McCaskie.)