Ask HN: How to determine your value to a startup

I was recently recruited to write a web crawler that will become the heart of a piece of software created by a new startup. They are talking about offering me 5-10% stake in the new company. I felt this was somewhat low, considering there's only 2-3 dedicated programmers and only 4 people involved in the founding of this startup. How can I determine and demonstrate to my new overlords (one of them is a longtime close friend) that I am worth more tahn 5-10% before we get started working?