Depends a lot on the specifics of what the algorithm is, what market it enables, how much competition there is, and my own ability to acquire the skills needed to turn the idea into a profitable business. But as framed above - I'm taking the minority position and founding the startup. The fact that someone is offering $10M for the algorithm indicates that they believe it enables a market that is worth many times that amount. Those markets are very rare - most people go their whole lives without finding one. So it's rational to stick with it and develop it as much as possible while retaining ownership.