John, hate to say, but it is rather subjective. When I get referrals from a customer and it is a large job, i.e. a new site or major overhaul, I usually give them one year's free web hosting, assuming I host their site. Otherwise, I generally give $50-$150, again depending on the job. A third option I have done is send them a gift card to a chain restaurant which is safe to assume is in their area: Olive Garden, TGI Friday's, etc.
Someone recently shared with me that at certain points different amounts of money exchange. In example if a job is less than say 500 you would accept everything up front or just not do the job. As the price rises you accept smaller/more increments over time subjective to the price of the overall project. There are going to be people who have the merit to be able to give you repeat business referrals and some people who just so happen to be in the right place at the right time though on their own merit. How would you deal with competent merit versus one-time-lucky-person lack of merit?