John, you're misinterpreting the prices quoted on Intrade.
The prices represent the percentage chance of each candidate winning the election and each individual contract settles at $10 for the winner. Romney is currently trading at 38 so you would be risking $3.80 to win $6.20 (getting back a total of $10) making him a +163 underdog.
Obama is currently priced at 60 (you have to put up $6.00 to win $4.00) making him a -150 favorite.
While its pretty much a done deal that Romney will be the republican nominee, there is an extremely slim chance something weird will happen which explains why Obama's price of 60 and Romney's price of 38 does NOT add up to 100%.
There are 2858 contracts listed between 60 and 62 on the buy side for Obama totalling $28,580 in volume so I'm not sure where you're getting this $120 figure you speak of. Also, its important to note that large traders are hesitant on putting up huge blocks of contracts as something big can happen at any time and if the news goes against their orders, they will get picked off.
If you want to buy or sell either Obama or Romney and are willing to pay even one point above the market, you would have no trouble getting filled for $100,000 worth of action within 24 hours.
To anyone thinking of loading up on Obama solely on the basis of polls copied and pasted by liberal democrats showing Obama having huge leads in the swing states, DON'T DO IT. There are tons of polls being taken across the nation ranging from Romney having small leads to Obama having as much as a double digit lead. When you do your polling in Boston, Massachusetts, its not surprising to learn that Obama is up by double digits.
Liberal democrats spend their days searching only for polls that show Obama with the biggest lead possible. Any poll they come across that shows the race being close they choose to ignore in a form of mental masturbation if you will. If Obama was ahead of Romney outside the margin of error in the crucial swing states, he wouldn't be trading anywhere near 60 especially being the incumbent.
If you think Obama is going to win the election due to your own research, then Intrade is definitely the place to bet as they have Obama listed at the best price.
This election is very close and will tighten up even further now that the GOP nominee has pretty much been decided.