...in exchange for TF2 (or Steam) items.

I was thinking of these prices:

Rough sketch = 1 USD (0.4 TF2 keys)
Lineart = 2 USD (0.8 TF2 keys)
Lineart + Colour = 4 USD (1.6 TF2 keys)
Lineart + Colour + Cellshading = 8 USD (3.2 TF2 keys)

I'm slightly unsure and may change it, but feedback's definitely appreciated!