As the Tumblr/Yahoo deal continues to be negotiated by press, and the world gears up for whatever is being announced Monday morning, Tumblr founder David Karp is probably having a very interesting weekend. It’s likely, in between multiple discussions with his board members and Marissa Mayer, that he’ll take a break, like a walk or something, to gather his thoughts.

On this walk (or jog or glass of wine at a bar), he will likely mull over two main outcomes. He could take Yahoo’s money, whether it be the $ 1.1 billion that the board is trying to approve giving him, or the more that he negotiates. Or, well, not.

If he took Yahoo’s money, he would join the Billion Dollar Exit Club — you know, the ranks of Kevin Systrom, Chad Hurley and Steven Chen from YouTube, the PayPal mafia, Tony Hseih, James Clark, Marc Andreessen, etc. He would be considered “successful” by the Valley’s ridiculous standards and everyone else’s, but not Zuckerberg successful, but definitely Michael Birch successful. Maybe he’d buy a nice house in Presidio Heights for when he has to be on the West Coast, and fill it with art and an apartment in Chelsea? [And maybe a vacation home for his family. And maybe a plane.]

He’d still oversee the Tumblr product at Yahoo, at least until his lockup expired, and maybe users would leave and maybe they wouldn’t … But the game would be over. The race would be in its cool-down period. Still, a pretty chill life overall. Especially in this economy. What would Kevin Systrom do?

Sell.

But with this, just like with the Instagram sale, comes a nagging, cloying afterthought: “What if Tumblr (or Instagram or _______) could have been the next Facebook?” And this nagging opportunity cost would grow even louder if Yahoo succeeded with Tumblr, finding a way to monetize its millions of eyeballs much like Google did with YouTube.

“Tumblr could have been a contender.”

It’s this thought that will lead to a “No” from Karp and his board if it gets nagging enough. And this thought is weighty — Zuck had it too when he was being courted by Yahoo, and we all know how that turned out. But what happens after the “No,” the fact that Karp will be challenged to build a real business on top of Tumblr’s scale, is daunting enough to turn that “No” once again into a “Yes.”

Can Tumblr turn the process of following other Tumblrs through your dashboard into a stream it can monetize with sponsored, story-style ads? Or find a way to cram ads into the notoriously independent, and risky, content?

Can Karp put on the big-boy pants, hire a Sheryl Sandberg character, and create a money-making machine? Because if he’s not sure, and he’s not ready for a long, hard, uphill fight, he should sell.

Look what happened to Groupon; still trading below its $ 6bn offer.

A billion dollars is a lot of money.

TechCrunch