My hunch is that you'll have to provide more than just another artist's opinion for the $20.
Consider additional value you could bundle up for the $20, and create a 'killer offer' that you believe in, and that provides excellent value for the artist. Play with the parameters you bundle into the offer to make it absolutely irresistible. A/B test the offer of course.
Some ideas for bundling additional value into the $20 price:
1. Offer to make it a video review, and transcribe it as well. 2. Publish both the video and transcript on one (or all) of your web properties with links to the artist's website and top social media handles. Get the artist who does the reviews to also link to the artist being reviewed. 3. Create a 'discovery' directory with listings of each artist who paid the $20 for their review - I'm curious to find out about these artists myself - even the ones you don't want to promote - the $20 is an effective filter for both you and I!
Cool idea, and I LOVE to see people taking some risks and innovating the business model for music.