I think it has to do with the perception of the quality.<p>The single biggest risk on sleeping with a prostitute is getting a (or multiple) STDs, or even HIV.<p>If you sleep with a street hooker, your risk of getting something is relatively high (even if you use condoms, and all that).  That street hooker that cost a hundred buck for couple of hours, probably sleeps with 20-30 people a week.
Talking about used good. This is actually really risky.<p>A high end prostitue, could get away with sleeping with only few people a week, and still make good money.  She probably is sleeping with only rich people, so the risk factor is different.
The chance of her getting something from her are a lot lower.<p>Plus I heard that the high end ones, look really good, compare to the regular ones.<p>The really high end ones, are the ones that act as a gf. They don't get paid directly, but basically splurge on everything they can using your credit card.
If you have seen the move "Priceless", it is a good one to watch: <a href="http://www.pricelessthemovie.com/" rel="nofollow">http://www.pricelessthemovie.com/</a> (mostly a comedy).  But the "gf" for the month, phenomena is real in Europe, and some countries in South America.