They don't have to pay for the room. However while some ladies do that. it is a lot more common to see the outcall rate listed a bit higher than incall.
Various reasons for that. Here's a few, and I'm sure some providers will chime in with more.
1) Some ladies have static incalls. (sometimes their own, sometimes shared with other providers) in that case they already have a flat fee they are paying for their incall, (by the week or month or something) so they don't save money by not using it. Not all providers work out of hotel rooms for their incalls.
2) Transportation. They need transportation to get to an outcall. Some drive, then need to worry about traffic, parking, parking fees etc. Some have a driver (who they obviously have to pay) Some use a taxi or uber (more fees).
3) Travel time: they may have to travel round trip one hour or even more, for a one hour appointment. Time is money.
4) Comfort and safety: For an outcall they are going to a strange place they don't know, in possibly a neighborhood they don't know, to meet some guy they don't know. In an incall it's "her" place (even if it is a hotel room) She may just feel more comfortable and safe in her place. She has some control. It also may help with having everything she needs with her, and not having to take her bag of tricks on the road and risk forgetting something.
Also at an incall she can greet you in lingerie or naked. In an outcall she has to get dressed.
Here's something I don't get though. (But I chalk it up to more provider math.
Incall/outcall
1 hour 400/500
2 hours 750/950
4 hours 1200/1500
I have seen rates like that listed and though I understand reasons for higher outcall rates as I listed above, it makes no sense to charge more each hour. Whether the higher rate is for transportation or comfort, once it is covered it is covered.
I have also seen rates likes these (below) and that makes perfect sense to me.
Incall/outcall
1 hour 400/500
2 hours. 750/850
4 hours 1200/1300
: