Personally with startups I would be more concerned about the future of the job than the intial salary. If a start-up is well capitalized and has a solid business plan and a better future they are more likely to pay better and the job is more likely to last longer than 6 months. Seriously below market rate without some other compensation usually means the business plan isn't there or the funding.
If the start-up looks good and still doesn't offer what you want in pay then consider negotiating for a pay raise in a set amount of time, say three months.
One offsetting thing about start-ups is that usually you really get to stretch your skills as they can't afford specialists at first. This can be very much worth the increased risk and lower salary of the start-up. I certainly used this to get the skills I needed for my current specialty and then moved on.
I have serious doubts that you can ever really know what market rate is. Companies don't pay the same for the same job to different employees and they certainly don't accurately report what they pay to anyone. Nor do employees accurately report what they make in salary surveys either.
Be wise though, don't accept a start-up job thinking you are going to get rich like the original developers for Microsoft. It can happen, but it doesn't happen to the employees of the vast majority of start-ups.