For your New York literary agent, there's a wide range. The lowest agents on the totem pole might only earn a measly $25k a year—if they sell only to smaller markets and are still trying to assemble a substantial writer list. The average agent, though, makes more in the ballpark of $50-$55k annually, and if you are lucky to hook up with a best-selling author or two, your salary can swell well beyond this figure, maybe even over $100k. Keep in mind that much of their income is based on commissions, which are usually between 5% and 25%. So the better the sales of the authors they represent, the better they do financially as well.
Now how about Hollywood literary agents? Like their New York counterparts, their salaries are also based very much on commission. It isn't always a bad thing, but working on commission does mean your salary is always in flux. Typically, an agent's commission for the auction-sale of a screenwriter's script or for scoring your client a screenwriter for-hire job (usually this means your client will be re-writing a draft of a screenplay that a production company with a deal at a major studio, like Warner Brothers or Universal Studios, has already bought) is 10% of the gross payment.
In order to figure out what an agent makes, you have to get into the real mathematics of agent economics. For example, say Agent A has a Client B, and Production Company C buys Client B's script at auction for one million dollars. Of this one million dollars (a big number), Agent A takes a 10% commission, which equals $100,000 (a smaller number). Unless Agent A works at her own boutique agency, Agency X where Agent A works, will take a cut of Agent A's commission to cover the expenses of running the major talent agency that Agent A works for (leaving Agent A with an even smaller number). But take off your rose-tinted glasses of optimism, potential agent-to-be. Deals in the millions of dollars happen, but only for a small number of writers, and of that small number of writers, many of them are represented by the SAME agent. And guess what? The agents who have these heavy-hitting clients NEVER want to let them go. So good luck. It's what you’ll need to find and keep your stable of cash-cow writers and bring you to the big bucks.
As if working your way up the agent ladder isn't tough enough, your salary as an assistant to an agent (no matter how successful that agent is) will be pitifully small compared to the amounts of money others in your industry make. Sitting at a desk, working at least 50 hours a week, a lowly assistant can make as little as ten dollars an hour!
But there’s always your tax write-off, assuming you've made enough to pay taxes. Most anything agents do for entertainment can be written off in your taxes as a work-related expense. Yippee!