Research what local accompanists are charging. Look at a whole range, from established collaborators to those just starting out. It will give you an idea of the local market, and what people are willing to pay. IMO, set your charge somewhere in the range of fees you have turned up in your research of local accompanists. Don't start low, start in the range and adjust as you gain experience and get a feel for the market. ***Here are some funnies to start with https://classicalmusichumor.tumblr.com/post/43036526797/accompanist-abuse-list-of-fines