By acceptable, I don't mean outstanding. Just something I wouldn't be ashamed to put on a business card or website. You typically award one winner, but you do have the option to select multiple winners and pay them each the full amount.
These aren't necessarily top-notch designs, and the designers know how 99designs work. I've entered coding contests myself knowing I might not win. I don't know every designer's motivation, but it can be just for fun or experience. Obviously, there are designers that are willing to participate.
How much did the 'acceptable' designers get compensated?
That's the problem with the 99designs. There isn't a scale of compensation for quality. Either the designer wins or fails with the rest of the junk.