I would suggest that you start getting into hotel shops by doing a couple lower end shops in your area first before you venture out for the higher dollar shops. You are right, it can run into hundreds of dollars and you want to be very sure you know exactly what you are both doing and getting into before you start spending a lot of money on the front end. Some are fairly easy, and once you do a couple, you'll have a good idea what is expected.
However, I've had resort shops that I thought were a major score to get my hands on, but in reality, I had so much to get done on the shop, I thought I was going to fall flat on my face before I just collected all the information they wanted, and that did not include writing it all up. There was the very real realization that I was among the exhausted hired help, and was in no way on vacation. That said, a good shopping partner can make those more doable, and I did not have that at the time.
As to reimbursement, I've done many such over the years and only had a problem with one. That one ran into months of back and forth before I got paid, and it was several hundred dollars. There is always that hovering over all mystery shops. You do need to make sure you can cover it if the reimbursement hangs up for whatever reason. I would also suggest doing a few smaller shops first with any company you are thinking about working with that would cost you money on the front end. You really do want to know how their shops go and what their editors might be likely to flag before you start pulling out your wallet for the big bucks.
How many legs does a dog have if you call the tail a leg?
"Four. Calling a tail a leg doesn't make it a leg."
-- Abraham Lincoln