Ok, I don't really know how much stringing usually costs in the US.
But I can tell you this. I used to string in Britain, and I can tell you that we charged at least double on pretty much every string you've got that we had as well.
Prince Synthetic Gut - £22.00 (about $35)
Gamma TNT - £28.00 (about $45)
Wilson Sensation - £30.00 (about $50)
Head Intellistring - £25.00 (about $40)
Wilson NXT - £30.00 (about $50)
So from where I'm standing, your prices look great