I would also consider doing a trade for something other than art (tack, supplies, a service, or some other item).
Otherwise, I charge $10 per hour and per subject. Here are a few examples and prices to give you an idea of how much I might charge for what you need. All of these prices are per subject.
Pencil or digital sketch: $7.50-head/bust, $15-full body
Inked drawing (line art): $10-head/bust, $20-full body
Inked & cel or smooth shaded: $30-head/bust, $45-full body
Digital painting: $45-head/bust, $60-full body
The above prices are general. Certain features or accessories such as wings, fancy armor, or surfaces with high detail patterns (scales/feathers/spots/etc...) may be extra. Backgrounds are extra depending but if it's a colored drawing I'll usually at least put in a plain gradient background with colors that compliment the subject.
I only do digital art except for pencil sketches which can be shipped to you. I might consider doing a watercolor or pastel portrait if you're set on it, though we would have to discuss pricing.
Here are examples of my completed artwork:
Thanks for looking!