Well, I´m not so into this asset selling stuff or other markets to be honest but comparing to other stores it really seems to be cheap.
But as you say hard work....I can just see it from my point of view as a graphic artists. These UI´s you are talking about can be made in about let´s say 2-3 hours with a good collection of layer-styles! it´s just copy and paste then!
12 bugs is a fair price for that!
I definitely need to sell some of my stuff as well, if it´s paid that good