That's a good rule of thumb if those three people are all cofounders. If two of the three are employees with no stock then $1,000 might be the better quote.
Usually, I base it off of the order of magnitude that I believe (or know, if their funding is public on CrunchBase.com) they have in the bank. 1-3% of free cash flow is more than fair and when they have tens of millions in the bank from their last funding round, that can be a substantial number.