I don’t think you are necessarily being unreasonable, but just having two houses doesn’t make you well off, it could actually be the opposite as they have 2 sets of costs!
As for the hotel, as others have said, whether it is expensive depends where it is and when. Yes, you may be able to get something cheaper, but will it be as convenient, nice, provide breakfast etc?
I meet my uni friends for a night away in the run up to xmas. We normally go to a town which is more or less midway between us, but it is very touristy and busy, even in December.
This year I was doing the booking in mid-Sept, for early Dec, and the hotels were selling out as fast as I was messaging the others about cost. All premier inn type hotels were fully booked even then. In the end we got 2 twins, paying £80 each, £160 per room. All of us are reasonably well off professionals in 50s, so not a problem but still not cheap. However, worth it as we like the town, lots of restaurant choices and we wanted to be within walking distance. We could have got cheaper out of town, but it is nice to be able to walk in, for shopping and then again for dinner.
Perhaps message your friend and ask her if she can suggest an alternative. As others have said, with Christmas she may just not have the money, or maybe she has limited access to funds if she is a SAHM.
But I must say, from experience being almost twice your age, there will always be friends who you go the extra mile for, and they don't seem to value the friendship in the same way. If the friendship is worth it, and given the longevity it probably is, then maybe modify your behavior somewhat in the future?