I just had the same thought and compared their median price with a sample of their hotels on Hotelscombined, which is the best source I know for hotel price comparisons.

Priceline's prices were always higher, but in varying levels. (once it was $333 vs $204 in another $189 vs $187 and so on).

My guess is that Priceline quotes the average price on their site for all the hotels in this area. This is higher than what we can see on HC, because they compare more suppliers and then reach better prices.

So, using PL median index tells us nothing about the potential accepted bid price.