Here is a whitepaper that talks about using CD-Rs and problems with them, 7 fatal mistakes. As a photographer using CDs to archive all my work, I found it very interesting.
CD and CD-R use entirely different technologies, and a CD-R doesn't last nearly as long, especially if they get exposed to sunlight. When I lay out my cash, I'm purchasing not just the enjoyment of the music, but the knowledge that I'll be able to listen to it for years. You don't get that with a CD-R.
I personally would be very annoyed to pay full price for a CD, only to receive a CD-R. It's happened to me before, and I won't do business with those people again.
Don't cheat your customers.