I have to turn myself in tomorrow and have a warrant served to me. I am being charged with theft under $500. Can anyone tell me what the cost is going to be?

Are you in the united states? If so, generally it is a 500 dollar fine for anything under $250 stolen. And I believe $1500 for anything above $250 and below $500.

will depend on where you are at,

around here you will have to repay the value of your theft and sometimes extra for their time and trouble,
You will have a fine up to 1000 dollars and probaton for @ a year

around here for minor crmes a good judge is the bail you have to set is close to the fine