Depends on the one you want. The brewery sels them directly. Ebay has tons of them, and if you look at history you can get a good sense of comps and market value. https://store.ommegang.com/SearchResults.asp?Cat=1827
The one I can get is the Obama one that came out in 2009. There are two on eBay for $400 and I think $250. I just didn't know if it was that rare or its just people trying make a lot extra on it.
I think that one for $400 has been listed for some time. I just got one in off of ebay in January for just over $50; description says it is from 2009. I, personally, never saw the Obamagang (Fed official name: Inauguration Ale 2009) tap listed on Ommegang's web store site but it's not like I looked at it every day either. To the best of my knowledge, that beer was only made once and was only available in limited distribution ("DC, NYC, Syracuse [?], Philly, Chicago and Boston" according to RB site). Just based on that, probably a lot fewer made/available than the usual Ommegang brews.
