It varies greatly between artists. Fullbody commissions usually cost more than a headshot or bust. Generally speaking, coloured and shaded pieces will also be more expensive.
So, it really depends on what you want, and who you ask. I guarantee there'll be a good artist who has affordable prices in regards to what you're looking for. Also, there's another subforum dedicated to art sales and auctions. That's a great place to get started!
JuniperW is right. It depends on the artist and what you want done. A sketch of a bust by a brand new artist? You could probably find someone willing to do it for free. A full digital painting by a MtG artist with a city scape in the background and a thousand spaceships in the sky? Mortgage your house.