I think price of you product should be set accordingly to first of all it's quality (which is not that simple as it may sounds - different people have different taste) and second of all how much time did you spend on creating it. I had this problem setting price for my own "sprite pack" but after looking through many stores on many different portals (with a lot of crappy and overpriced stuff) and consulting with many friends (who says that what I've set is way to low) I think my price is quite acceptable and I'm happy with it.
And there's already few good assets on new store that are quite acceptable regarding to quality and price.
Besides that you must have some dignity and respect for your own work. Spending few minutes in photoshop, illustrator or whatever to make few shapes in different colors, and wants 50$ for that is... let's say it politely, not very nice.
I know it's still very early for the store but I would like to see some more not-pixel-art assets