I also agree your second piece is your best work. I think it's because it looks more proportional whereas the chibis look a little off balanced; I think it may be because their foreheads are too big. I do like the shading on the first one, though. I think maybe making the pupils more defined like your first and second examples look more pleasing. It seems like the third one's eyes blend in too much. ^^;
I agree with Pokemanz's prices of 200-300, probably even more. Maybe try having people present offers and get an average of what people would want to pay to get a feel, first? Hope that helped a bit!