I have no issue with CSG graded cards. If they're cheaper then I've picked up a bargain. I can't answer the overall discount because that needs a fair bit of analysis, but I'd love to see it if anyone has that level of research.
CSG does seem like a good opportunity for bargains. I'm sure there's people out there who'll make a living by buying CSG graded cards and cross them over to PSA to profit from the difference.
But it doesn't always seem to be the case. Here's an example that I found after a few mins of looking.
56T Mantle PSA 2 White Back
56T Mantle CSG 2 White Back
I'd love to see a more exhaustive analysis.