Seeing as you'll be able to use those recorded accompaniments literally forever, you should probably bump up those fees a bit. I'd at least double them, because you're taking away future income. If you have a recording of a solo, are you going to go out of your way to hire someone to play it in the future?

Also, consider: you're not paying for the 5 minutes it takes to record a solo, you're paying the player for the 20, 30, or 40 years of practice it took to get good enough to record those solos in 5 minutes too.

That being said, there are people who can get you piano recordings of many of these solos. Ask around a bit - I know there's a Google Drive/Dropbox on here that a guy can provide you with that has hundreds of solos pre-recorded.