Interesting thread...
I was talking to a buddy recently who referred to MP's as a "gateway drug." You get to the point there SC's aren't enough, so you move to MP's. Then when that isn't enough, you move to SP's. He had an interesting theory, and I'm sure that there are some guys that fall into this trap. However, at the end of the day, I think I'm pretty smart with my money, and there are lots of times that I go a month or so between visits to MP's, then I might go a couple times a week for a month. If you have any degree of intelligence about your cash at all, you realize its all relative to what you have and what you choose to spend your disposable income on.
Personally, I don't think I will ever "trade up" to an escort. Its not about the money (albeit that is an issue... I can justify $120-$150 at an MP, but not $250-$300 for an SP). Its because I like MP's. I feel safe, I don't worry about what the neighbours would think seeing a woman dressed like a school-girl showing up at my door, and somehow a SP seems more "wrong" to me ethically.
Just my 2 cents...