I think there's definitely a market for helping artists get discovered, especially as more and more of them tend to operate online from their bedrooms instead of going on tour. So kudos for trying to help them along on that front.
From the other comments here, it looks like the $20 goes to the "feedback", and not the promotion to labels.
From an artist's point of view, this seems backwards. Personally I can't see why I would pay $20 just for feedback on a recording. If it's a demo recording, then there will be lots of things about it that could be improved, but I will already know this. If it's a professional recording from an album I've done, then it's sort of too late for feedback on it to do much for me. I suppose I could take it into consideration going forward, but my ability to act on this information still seems pretty minimal. So it's not clear why the "feedback" would be worth my $20, especially since there are lots of free ways out there to get feedback on your music.
"Submit your music for free, and if we like it we'll contact you about paying to have it promoted (and maybe give you some feedback)" is a model that would make more sense from my POV as an artist. That said, I know it's not the easiest model from your POV, as you'd be inundated with YouTube acoustic guitar covers.
So my advice would be to work on making it clear why the $20 will be well spent. I'd be more likely to pay up if I knew how likely I was to be "liked" by you, how likely I would be to get in touch with record companies if this were to happen, and exactly what benefit I could get from your feedback if I wasn't selected. This might something that gets easier once reputational effects start to play in.
Hope that's helpful!
P.S. My band, in case any one feels the need to investigate my boasting: http://doctorsquid.bandcamp.com/track/on-my-way