I honestly think it would be pretty rare to find a hobbyist that didn't have preferences. If a person just wants to fuck whatever moves-- they could just pick up any old slag at the bar... most gents who hobby-- do so to have the security of not having to call them or see them again if they don't want to-- and to be able to perhaps spend time with a lady that wouldn't be so easy to attain in the "real world" (at least, not without a considerable amount of time and effort). Sometimes its more of a matter of looks. Sometimes its more of a matter of personality. Sometimes its a mix of the two.
As far as a rate goes-- I've always loved the saying "You're not paying her for her time, you're paying her to leave at the end"