It sounds like you mean actually owning the property the practice is on. In that case, averages mean nothing; there's too much variance in real estate prices from one location to the other. You could buy thirty buildings in Podunk, Iowa for the price of a little shack in New York City.
A small to medium-sized practice - patient lists and used equipment only - usually sells from $200-400,000 from what I've seen. Though this too will vary with location.
Just for an added example, my partner and I are in the final stages of determining if we're going to move to a new building or add onto our existing facility, the final two options we're looking at are as follows. Granted all the property/cost info are in rural CT, so for an urban setting you may very well have to add some $$ to the number, or for a western rural setting, you may take away some $$.
Basically we're going to be going from a 5 operatory office to a 7 operatory setting, with buying all new equipment for 4 operatories and using the existing equipment for 3 of our current 5 rooms. Currently we have about 1600 sq. ft. on the main floor of our practice and an open basement for storage below.
If we move to a new office, we're looking at about 3100 sq. ft. currently basically open space on a poured concrete slab. Real estate cost would be about $175,000 and then the renovation/construction/equipment/installation would be roughly another $175,000 to $200,000.
If we add onto our existing office, we already have zoning approval to add another 30ft x 15ft to the existing space taking us up to a little under 2100 sq. ft. on the main floor. The estimates we have for site preparation (including the removal of 40 linear ft. of retaining wall, excavation of approximately 50 cubic yards of earth and fabrication of a new retaining wall - the back part of the property that my office is on is carved into a hillside), foundation work/construction/equipment/installation is in the $200,000 to $225,000 range.
The biggest deciding factors are #1 do we want to move the office across town (about 1.5 miles down the road), and #2 when do we want to do it(do to availability of the contractors we could be in the new building within 3 months, while adding on wouldn't have the new operatories ready for close to 6 months)