The good news is that DPC practices are similar to other industries with recurring revenue. You probably won't have much luck with typical medical practice sales models as the doctor is the only value. DPC puts the value on the book of business, depending on your cash flows. Very easy to value by looking at historical cash flows, applying a growth rate for future years and then discounting the projections for present value.
For example, using an APV model, and assuming a few things on interest and borrowing rates, a business that has generated a net income of $100,000 for the past 10 years and can project 2% growth in the future has a value of about $1.3 mm. With a future growth rate of 6%, you're looking at a little over $2 mm according to some models.
If you don't have profits or you take them out of the business, use total revenues instead or EBITDA if you have interest or depreciation expenses.
Lots of variables involved, especially dealing with patient relationships. Lots of good info and models and spreadsheets available online.