It's not very difficult at all. The best place for this is HDR/BM/KL. They are indy's so they have no prior obligations locally, except for their Babi*s, so they can usually go away with little notice.
#1. Do not just pick a travel companion based only on looks or sex, you will be with her 24/7 so you will need to be compatible out of bed (i.e. don't pick a girl who doesn't speak english at all, if you don't speak spanish at all.)
#2. Spend some time (several short sessions, maybe a longer session too if possible) with potential candidates before asking them to go with you. Negotiate things upfront (i.e. # of times a day).
#3. The fee... this is very subjective. I would ask her for what she expects, then go from there. Because if you say $200/day when she is expecting $100/day, you just screwed yourself out of $100/day. You want to get a ballpark of what she expects, and you don't want to price yourself out of the market or overpay. If she says $400/day and you were thinking $150, you probably shouldn't waste your time.
Personally, I think $150/day is reasonable, but it depends on your budget. Not that the chicas care, but getting there/back costs money, the hotel costs money, and you will feed her the whole time, so you will end up spending a lot more on her than what you hand her at the end of your trip together.
I read on another board a guy said he found a "hot" girl from HDR to go away with him for $35/day.

I would pay to see this hottie.

This same guys says he pulls chicas out of HDR for $30-40.
