I think you need to help less, waaaaay less. I understand. I do similar things when my parents come to visit me here, mostly because my mom asks/expects it. So I get it.
But really, their experience/holiday is going to be good or bad or inbetween regardless of if you help them. And if you DO help them, and then it sucks, they will blame you, etc.
They are grownups with internet, and unless they ask you for specific help (which hotel, this one or this one?) you should leave them to it, as it will only bring you grief and stress.
As to 'why no luxury spa' or 'whys Boston so expensive', I'd just nod and smile, and fake commiserate. 'It's such a shame, yeah I know - things are so expensive.' Cut your losses - you're not going to convince them and at least they'll like you in the process.
Don't waste your time on them, honestly - it'll only end in tears. This is the beginning of the rest of your in-law life - start now separating their contentment from your efforts.