Connie, are you looking for cash, or labor? If cash, where is it coming from (job, disability, welfare, college housing voucher)?
You might want to try looking in the newspaper, and see what people who are offering room and board in your area are charging, and then figure out what is right for you.
Are you talking about charging a relative? Or is this just someone staying with you? We charged both DS's after they left school and had a job. We based it on their salary and what all was to be included in "room and board". Then when they moved out on their own we gave them all the room and bored monies they had been paying us. They were able to use it on deposits for ultilities, apts, and such that way. Thye never knoew we were going to do that so they saved up for those things themselves; this way they had unexpected cash to work with.
Check your local papers to see what rooms are going for in your area, that is what I did when ds1 got his full time job, depends on what you're offering with your room and board, food or washing or .... as to the final price. If it's not family state rules, you may not wish to offer a non family member meals or whatever.
Don't stay were you are tolerated, go where you are celebrated! 
