If you're afraid of losing a lot of patients when buying an existing practice, a lot of people do this: Find a practice you love whose owner is about to retire. Then you work there for a few years as a partner (or even associate), but with a contract that says you will purchase and own it within a few years. That way, patients are introduced to you slowly and get to know and trust you. Also, practice transitioning is a lot easier that way, especially if you are keeping the staff and general layout. You could also help earn money towards the purchase price.
I've heard around 300-600K myself...totally depends on what the practice brings in (and how much the dentist wants to sell!)