I can't give you a figure, but I do encourage you to consider not only your current traffic figures, but also potential traffic figures that will result from your site being introduced to the acquirer's user base, if that makes sense in your scenario. If they get 10x more pageviews and uniques a day and they give you a prominent position on their site, your traffic could instantly double.

I guess what I'm saying is that the synergy that comes from an acquisition (or any strategic relationship) can sometimes have a value all its own.

And there's almost always an emotional component to consider that supersedes spreadsheet models. If you enjoy running the site, then it might be worth more to you than if you don't.

Best of luck!