@Tamsin4 While I am dismayed by what seems to be a lack of basic manners these days, in that a guest can't be bothered to spend 30 seconds thanking a host for their reply, and this business of sending out inquiries to tons of places, angling for the best deal, is disrespectful of hosts' time, it helps to think of it as just part of the job of hosting.
How many people go into stores and are attended to by a salesperson, only to walk out without buying anything?
The difference is that the salesperson is getting a salary to attend to customers whether the buy anything or not, although some in some businesses the salesperson might also earn a bit of an additional commission on the sales they make.
So we just have to charge enough so that the directly unpaid time of answering inquiries that don't turn into bookings doesn't feel like an imposition.
And maybe create some generic saved responses that can be personalized as necessary, rather than taking a lot of time to answer each message in detail.