NOTE: As of the time of writing this, I cannot accept money.
clean sketch = $5
lineart + flat colour = $7
full shading + highlights = $10, 1 month nitro (minimum for nitro payment)
extra: airbrush rim lighting = additional $3
abstract = $2
simple scene = $5, 1 month nitro
polished scene = $15, 2 months nitro
additional characters = $7 (or 1 month nitro) for every additional character
note: all of these are rough estimates assuming the art is a simple character art. If you ask for something more complex, the price may vary.