Sure, but you are assuming "serious money", unfortunately there are lots of people out there that can easily spend a week choosing an item in Amazon.com worth a few dollars but waste tens if not hundreds of thousands of dollars worth of savings in an instant on some get-rich-quick scheme which a prudent person would stay miles away from.
My point is, if you want to regulate a market so that it's being tradable as a market and not a casino, 7:1 should be more than ample.... because serious people don't treat it as a casino and they very very rarely use more than that.