You presume correctly, I found them at NH Counties Registry of Deeds.
(Thanks Just Sold for pointing me in that direction!)
We were wondering about the money ratio from then to now too - thanks for the push.
This Consumer Price Index calculator (CPI) states that $800 in 1853 converts to $19120.80 in 2005.
Not a bad deal at all!
I don't recall that Winnipesaukee Steamboat Company did anything with the island of historical significance either. Perhaps they were thinking of using it as a tourist destination? It's a beautiful spot.