Here's a handy infogram to show you what you will be receiving in the various tiers. I did get rid of a couple tiers since there didn't seem to be any interest, but as you can see, the $15 offers the most bang for your buck. I do plan on getting a few more color images out before the end of the month.
For the $20 tier, you will be getting that bonus sketchcard!
For the $50 tier, you get the $15 rewards and the commission for the next month!