I'm not actively in business these days. I did some CD pre-mastering a long time ago. These are a few basic things I learned back then. Hopefully, this will get the discussion started and more qualified people will give you more current and relevant advice.
For starters, check out studios and people that are doing the same services in you local area. Find their web sites and see what they're charging. This will give you a starting point.
Once that starting point is found, decide on a strategy.
Do you have some unique talent, service or equipment that lets you justify charging more than the average studio in your area?
Do you want to charge less than the average in the hopes that you'll get more business?
You should also spell out exactly what your client will get for his money. Charging by the hour is probably your best best. If you charge by the song be prepared to work some song that takes forever and you'll still get the same price as a song that only takes a half-hour. If you charge by the hour, you should be able to give your client an estimate on how long it will take to do the work and how much he should expect to pay. If something will take longer than the estimate, you should get approval before you do the work.
Some negotiation as you get started is OK, I suppose. Don't sell yourself short, though. It's still your time and effort.
Hopefully, others with more experience will have more to offer.
Good luck.