That's because most common dental procedures are relatively straightforward and the mechanical processes of the mouth are relatively well understood compared to most of the rest of the body. Plus most dental procedures are elective in the sense that you can usually just have an offending tooth pulled if it gets too bad, so for most people there's a price ceiling that is much lower than say a lifesaving heart procedure.
Why can't a hospital tell me how much an xray costs then? If I come in with a broken arm, why can't they tell me how much it will cost to get it xrayed yet the dentist can do the exact same thing with a broken tooth?
A dentist office is more comparable to an urgent care clinic than a hospital. And in many cases an urgent care clinic can tell you what simple procedures will cost. There are imaging services companies that can tell you upfront how much diagnostic tests are going to cost as well.
A broken tooth is a much less complex injury than a broken arm. In many cases you don't even need to be a dentist to diagnose it. Try walking into a dentist office with a more complex injury that could involve multiple different dental specialties and severe underlying complications. You aren't going to get an immediate upfront price.
1. Dental x-rays are more straightforward and involve fewer people, so the price is easier to calculate.
2. Dental x-rays are more often than not elective procedures. It's much easier to comparison shop elective procedures than emergency procedures. Therefore market forces frequently compel the dentist to come up with an up front cost for an x-ray.
The same market forces don't apply to hospitals because you can't wait a week or more while you shop around for treatment for a shattered hip.
