You could say some variation of, "Sounds good. Let me know how much my room costs/what I need to budget for my room/what I owe for my room."
I think this makes it sound liek she is offering to pay, and he can respond with "Dont worry about it." And then what - what does that mean? That he's only booking one room? That he'll pay for both?
I wouldnt leave it open to that much interpretation. I also wouldnt be going on an overnight with a guy I'd never kissed, and leaving him to make travel and accommodation arrangements for me.
OP, I'd urge you to make your own arrangments. Ask him for the name of the B&B. Tell him that it sounds great, but you'd prefer to make your own arrangments, can he send along the name of the B&B? You'll book today, so he can book as well (or whatever line assures him that you'll in fact be attending).
This is going to sound mother-ish, and I apologize in advance, but your post sounds like you havent reached a level of intimacy with him yet where you feel 100% comfortable. And give that fact, I urge you to think about your transportation there and back, your accomodations, what you're willing to accept and what you arent, and spell it out clearly for him.
Edited because I re-read the OP, and not all of my post was applicable.