I can't speak to average hotel rates in Phuket, since I'm no expert. But I did a fair bit of searching around for options for an upcoming trip to Phuket that my partner and I are doing (Dec. 3-8). What I found was that there were plenty of cheap options, but the nicer and better-located properties were averaging between $150-250 a night (with the higher end of that scale at the real luxury properties popular with Westerners - and a few of the super-luxury spots charging well above that).
The comparison with the U.S. seems odd. I'm not sure where the OP is staying in the U.S. and getting rooms for $50. Maybe Des Moines? Fresno? I travel often to big cities in the U.S. that are popular with tourists and I'd say I pay $125 or more for a clean, but simple hotel in SF, $200 or more for something in NYC, and recently paid $149 and was thrilled to have it in Philadelphia Center City. These were all perfectly fine hotels, but nothing fancy. In comparison, the hotels in that price range in Phuket seem to have large rooms, include a lavish breakfast, are usually on the beach, and have spas on property, etc. A room of similar quality in a major U.S. destination would be $300 or more. So let's compare apples with apples. Phuket may be expensive for Thailand, but it's still cheap for the overall quality/location of the properties, from what I've seen.