Depends on the size.
Mini (50x50) (like the ones given for free earlier) - $1
Small (100x100) (the same size as above) - $2.50
Medium (200x200) - $5
Large (300x300) - $7.50
XL (anything above 300x300) - $10
Considering it takes me about an hour to do minis, and multiple hours for anything bigger, it's less than minimum wage.
But for now it's relatively cheap, because people's requests give me inspiration and I enjoy pixelling. The money is really just for a little extra incentive for me to do it quickly. ^_^
The shop for them is on my website.