I would say, generally speaking, that the doctor (the guy) pays for the hotel, or that the cost is split between the doctor and patient.
The reasoning behind this is simple: there are simply more men wanting to play doctor than there are women. Economic principles definitely apply here. A greater supply of available men equates to more choices for the women, which in turn increases competition among the men to present more attractive offers. This explains a number of things--why some men write stories, create private sites, post pictures of play spaces, post pictures of themselves, create medical history questionnaires, and all sorts of other things. It is, for the most part, about selling themselves effectively to the limited number of willing women. Some women might offer to split the cost as a matter of virtue, though.
Now, follow-up visits are a whole other matter entirely. After the first "appointment" has been completed and both parties are happy with how things went, the relationship solidifies. The man no longer has to worry about competition; the woman is usually content to play exclusively with him, or at the least, inclusively. At this point, I would have to assume that the cost becomes more of a shared thing, where as the hotel cost of the initial visit would probably be paid for eighty percent of the time by the male. Also, as the two become closer, they might decide to play at each other's homes.