I saw a question on Facebook where a person wanting to get published was looking for a literary agent. However, the future writer hadn’t written anything yet. To get a literary agent, most agents are going to want to see at least one book and probably several before deciding if they will represent you. Most good literary agents are already booked to the max with clients. Short of a good literary agent having an author quit writing or die, you are stuck with those who may not be as good who are available. Some will be good, but many are going to be average at best.
Personally, I won’t be going with a literary agent because I plan on self-publishing. The odds of being picked up by a traditional publisher are very low without a good literary agent. Plus a literary agent will get a cut of money from your books. From a quick search, a literary agent often charges 10 – 15% commission.
Using this link – https://www.thebinderyagency.com/blog/howdopublisherspayauthors for expected royalties an author may see; if you make it big enough, you will probably do somewhat better:
Royalties paid on the retail price are fairly simple to calculate, and unless, the retail price of the book changes (which does happen), retail royalties are locked in. Average retail royalties fall in the 10% – 15% range on Hardcover sales, and 5% – 7.5% on Trade Paperback sales, generally.
In the above scenario, it doesn’t include the percentage you will be paying to your literary agent. For paperbacks, your margin is already thin enough without having to pay an agent.