It's a good idea to get good solid market data, but you're going about it the wrong way:

1. "Monopoly" is a trademarked and copyrighted property owned by Hasbro's "Parker Brothers" subsidiary. There aren't going to be very many "Monopoly-based" games out there, simply because you can't market such a thing with that phrase without being sued to buggery and back by Hasbro. Besides, Hasbro have pretty much cornered the "Monopoly-based game" market by selling, er, "Monopoly". At best, you might carve off a tiny, tiny crumb of the market, but you'll have a steep marketing hill to climb. More like a cliff, really.

2. Are you seriously expecting rival businesses to give you commercially sensitive sales information so you can then enter their market and take some of their profits away from them? Good luck with that tactic.

There are other ways to get relevant information. By far the best is to subscribe to the relevant trade magazines, which are usually full of sales figures and market statistics.

Well, not exactly a monopoly game, but here's some sales stats for Democracy game. There's also other sales stats, but I suppose Democracy goes closest to what you wanted.