I recall fondly my night on the Mont in one of those little hotels. No, they're not 5 star luxury, but I found nothing wrong with ours, and I've slept in many funkier places. Of course, if they were anyplace else, they would certainly be overpriced. But I thought my money was well spent. On Location Location Location. I haven't stayed at any of the places on the mainland so can't offer any input on those - other than I loved being on the mont during the empty hours.
In fact, I thought one night was perfect. Arrive mid- to late-afternoon. Enjoy the diminishing number of people until that dwindles to just a handful overnight. Hit the sack early, rise before the sun, be out to catch the sunrise. Enjoy the morning, and when the human tide rolls in, get outta dodge. No need to stay more than 1 night IMHO.