Suppose you're playing live poker, you've just successfully pulled off a huge bluff and the villain immediately says to you "I'll pay you $x to see your hand".
How much should you charge the villain to flip over your hole cards and show your hand?
I've been offered 5bb ($10 at 1/2) to show my cards sometimes after a huge pot. Sometimes I will pay to see someone else's cards too, particularly if it was a huge pot and I couldn't tell if they were making a fancy play with pure air, semi-bluffing with a draw, or had a genuinely strong hand. I feel that it's sometimes beneficial - especially for new players who want to see how the regs play - to pay to see someone's hole cards and then review the whole hand and try to understand why they did what they did.
If it's a decent offer, say 10+ BBs, I'll show. If it's a small offer, like 5BBs, I might say something like "You can pick one card. Double it and you can see both."