Interesting idea, I could definitely see a market for this. Although there's some things I can't help but wonder, let me elaborate.
First question I have is "Why would bands want do do this?" In the case it's just "Meh, just wanna hear what I have written with something that at least resembles real instruments" - In this case, you could just download Shreddage/whatever, Zombass/whatever and Superior Drummer/whatever, learn to program it well and it will sound pretty convincing. Would save you an awful lot of time (at least in more technical songs, which in all honesty, Guitar Pro songs tend to be), you wouldn't have to buy new strings and the band would be happy, given that was the reason they wanted you to do it.
The only other reason is if they actually wanted you to RECORD their song for them (which I guess was your initial thought), so they can release it. Which is also a very good service, but then you probably have to count in mixing and mastering. And this is where your price gets REALLY low. Because this is, as I have understood, what you are selling, the "whole" package, recording/mixing/mastering.
Don't want to bash on you man, just wanted to give my two cents. As I said, I think this is a very interesting idea. I have actually thought about doing this as well, but with vst's only!