There's no hard and fast answer to this one, and it really depends on the company, the industry and the candidate.
My personal experience in joining a Mobile Software startup (9 years ago, company was < 2 years old, had a total of < 20 employees) was that I was underpaid by about 15%. I was a fresh college graduate (MS in EE) and compared my salary with close friends in my graduating class who took jobs at Yahoo, Microsoft and Qwest.
What I did get was a bucketload of options.
In most cases, I've seen that people are underpaid - after all, you work at a startup for the opportunity to get rich quick, and being underpaid is the opportunity cost.
In your case, the most important factor is probably that the startup doesn't have traction (assumed to imply "is not profitable yet"). If they don't have money to spend, it's unlikely that they will be handing out generous salaries unless you are uniquely equipped to turn that situation around.