This mostly depends on your location and what the market will support. In densely populated urban areas there is more demand, but also more competition. Your experience and reputation will also factor into what you can reasonably charge.
Most Accompanists charge hourly rates for rehearsals and performances, but depending on the gig, there could also be a flat fee for rehearsal and performances combined. Ultimately, what you charge is up to you, but it needs to match the market conditions if you are going to work a lot.
For entry level, let’s say a recent college graduate, you should be able to earn between $35.00-40.00 per hour. A highly experienced Collaborative Keyboardist might charge $50.00-75.00 per hour. Factoring in several rehearsals and a performance, this might translate to $300.00-500.00 from a single client. Working freelance, a good player might be able to earn several times that weekly, while someone starting out would earn much lower on the scale.
Some top performers might charge $100.00 an hour and receive several thousand for a series of performances. Keep in mind that many concert halls and some schools will require you be a member of a union, which may have some power in setting rates, although union rates usually fall below what you could negotiate on your own.