I really have no specific explanation for it (that's really not the crew I hang out with), but exploits for the Mac ecosystem, iOS in particular, are in very high demand and do cost more.

Some theories:

I don't necessarily think it's because Mac 0-days last longer, but I know they do consider it a challenge; The difficulty may factor in to the price somehow. Maybe because there's the presumption the average user on iOS may have more cash to burn than the average Windows user.

They may have more to gain by hitting Apple employees for trade secrets.

Of course, all this is pure speculation.