I bought it around January of 2011, when I knew I was going to move forward with the concept. It seems kind of tacky to share the amount, but it cost about 5-10x less than what most people guess, as the previous owner was extremely reasonable.
IMHO, it does not seem tacky at all to share the amount, especially as a fellow hn reader explicitly asked to know it.
Obviously, you have every right, no to disclose it, for whatever reason.
The rest of the world would still like the data point.