What is a polite but assertive way to turn down a job offer from a startup company because of their weak finances?

I go with the direct approach.  "I'm sorry but I'm not interested."  If you get tons of e-mails and calls then just ignore them. One thing that I've found is not to be concerned with hurting someone's feelings.  If you want someone to go away, then the best approach is to be harsh and honest and to hurt their feelings.

Most of the times telling them that you are already involved in something works fine. If they still insist then you should name your price and they will take a backseat.

Thanks to the world of social networking, the last thing you want to do is to burn any bridges, no matter how inconsequential you think they are. The person you upset now may be a person who can help you find a new job down the road. That aside, let me share with you a dirty little secret about those industry group meetings. Many exist to match business people with technical people who have similar aspirations. In many cases, there are technical people who are more than happy to shoulder the risk, forgoing an immediate paycheck to get a bigger payout at the end. Sure, there's no guarantee that will happen, but more often than not, it seems pretty likely if you have a good team with passionate people. Given your question and your tone, I am guessing that you are really young, very clueless or both. The fact is that the person who came up with the idea is just as important as you, if not more so. As someone who actually goes to these meetings to meet technical people (and hire them), if you gave me 1/100000 the attitude that you shared here, not only would I not hire you, I would tell my colleagues to avoid you like the plague as well. Remember, it takes two to tango so to speak, and I have encountered plenty of arrogant engineers who think they know what they are doing, only to have me blow them out of the water with elementary questions that they should know the answers for. Long story short, I think you may want to stop going to these events. It may be safer for you to stay in your safe little cocoon where you can code all you want, go home, play your computer games or do whatever you do that you enjoy doing. Leave the networking and the start-up stuff to those who actually are interested in it. After all, if you keep going to events like this and continue to cop an attitude, you won't be getting requests like this, or for new paid jobs either. Good luck!