Does your dad/mom own the real estate too? That would be a separate transaction if so, and can sometimes wait till later down the road, but should be addressed as a right of refusal or right of offer for a period of time if/when the new owner wants to purchase.

The sales price is typically either a multiple of EBITDA (as low as 3x as high as 19x...Thanks, Mars!) or a multiplier of revenue (0.8x - 1.25x). It depends on a lot of things including geography, growth potential, size of clinic (other associates?), lease term/rate, fixed equipment (owned or leased), etc...

If selling to your associate, it will usually be done partially owner-financed, and with a bank note. BoA, WF, and Live Oak Bank all have good vet practice lending programs. Final sales price will have to work with cash flow to ensure a debt service coverage ratio. Some of these have fixed programs, others have SBA lending which is floating rate, typically LIBOR + 200, plus or minus. Your parents' clinic's bank may be the easiest route, especially if there's an established relationship.

Your mom/dad should also be expected to hang around for a transition period (6 months - 2 years), and have a hefty non-compete to prevent him/her from starting up a new practice in the same area.

Let's say you can come up with an EBITDA of $250k and a 4x (using these for the sake of easy math) multiplier is normal for your area, so sales price of $1MM. Look for something like this:
$100k cash at close from Associate
$150k owner financing (5 years at 7% using here, $36k/annual debt service)
$750k lender financing (10 years at 6% using here, $100k/annual debt service)

That would get you a debt service ratio around 1.8x, which most lenders would accept.