If you do a search for threesomes you will find that, as ever, different people do different things.
Some charge double, some charge more than that (to discourage them), some charge less than double, some charge per hour regardless of whether it is one client or two. Someone who used to post here did a mix: normal price if they were sexual with each other, but double if it was two straight men.
What makes you go 'yeah, that's enough'?
I will say that for a lot of people, her being "apparently" interested wouldn't be enough to consider the booking in the first place - they'd insist on talking to her.