I'm a little surprised they're not taking bets on whether there will be more protesters or inauguration attendees. I'd put money on protesters.
Counting heads is tough. The park service used to make an eyeball guesstimate, but stopped after the Nation of Islam threatened to sue after NPS said the Million Man March wasn't. The biggest crowd appears to have been around 1.4 to 1.8 million eight years ago. Of course, it's not obvious whose side anyone is on in a satellite photo. There are several other silly prediction markets: PEOTUS tweet count for the week ending today (99¢ on 45-49) A post-speech victory stroll (84¢ on YES) Clinton mentioned in speech (81¢ on YES) Obama mentioned (91¢ YES) Putin mentioned (79¢ NO) China mentioned (52¢ NO) Hacking mentioned (75¢ NO)