I would definitely consult a real estate attorney. You'll definitely want to get all the specifics including how the earnest money is to be handled in writing.
Now, I'm not so sure I agree with Ed's comment below, you may save some, it would depend on how many revisions your attorney has to make and whether or not the buyer chooses to have an attorney review the contracts yours generates (if you're a buyer I would absolutely advise this). You'll want to sort out in the contract who is responsible for those fees. Any attorney you retain will be working with your best interest in mind and will default to giving any benefit in a transaction to you. That's why I advise buyers to have their own attorney review the contract.
The attorneys I work with bill at an hourly rate and I had a lease purchase contract drafted and it was approximately $500.00 for 2-2.5 hoursâ€™ worth of work for him.