Presumably with a startup you're betting job security and money for a chance at bigger money down the road. If you make up numbers for IPO/acquisition value, what percent of the company you might own at that point (be sure to understand dilution) and how likely the company is to succeed, you can at least calculate an expected value.

You also get to work in a different environment, where there's very little in the way of existing turf, and you're welcome to take on as much as you want and prove you can do it. The things you learn and the people you'll work with may well make the experience worth it even if it doesn't pay off this time.

When looking at a startup, by far the biggest thing to consider is the team, specifically the founders. The business plan will almost certainly change, the product will change, but the team is going to be about the same.