I always considered absolute minimum to be about N x 4 x 10^5 dollars (where N is number of founders). This is just because $400k is enough to support my current lifestyle on interest alone, and would allow me that much-dreamt-of freedom from the tyranny of bosses if I choose.

The site I'm working on is more niche than broad, so my sale goal is N x 2 x 10^6.

However, I think that I am strange in really hoping my site actually generates cashflow once it's up and going, and I don't have to constantly look for funding or an acquirer.