If it sounds to get to be true, it most likely is. I've never heard of anything like it before. I also do not think that one keyword is going to make your website go to the top of Google, or anywhere near it for that matter, unless it was there before then.
If you do decide to do it, say you want results before you pay them.
If you decide to give them your credit card number before you receive anything at all from them, be sure you look online at your statements daily, if not hourly, and be prepared to cancel it.