If so, why would somebody spend months working on a IE10 exploit (http://threatpost.com/en_us/blogs/pwn2own-browser-exploits-g...) and then demo that here instead of selling it in the market?

If the market pays lots of money for Mac OS X exploits, why would it pay less for Windows ones? It can't be market share and I doubt it is because Mac users have faster Internet (so that their machines can be bigger DDOS sources, have more money to steal from them (both may or may not be true, but I doubt that fully offsets the difference in market share)

Another only thing I can think of why Mac exploits would be more expensive is that buyers expect Mac zero days to last longer, but I doubt that, too.

That leaves two reasons: because it is so easy that nobody considers it a challenge, and everybody expects someone else to pick up the price, or because it is too hard.

Alternative theories welcome.