- People are risk averse.
- Most hackers make pretty good money at 'real jobs'.
- Startups are definitely a risky proposition. As someone else commented, "A startup founder with a successful startup is already a rare exception.".
- So when you're in the thick of it, and there isn't a lot of money, and you don't know if it's going to go anywhere, that reality is going to wipe out any talk of "we might be rich", compared with the alternative of "making pretty good money and living comfortably".
The "this is what makes me happy, how can we both make it work" is the approach that has best worked for me. My wife likes coming home and finding that I'm so much happier than when I was working for a moron, even if there's no money in it (yet).
I know it's a chicken & the egg problem, but demonstration of actual 'entrepreneurship' is my suggestion. The best way would be to get a demo or product working ASAP and get users, fast. You have a better chance of arguing a case with something more concrete than a prospect.
If this has to be done while you work in another job, the 'Schachter' model (work, expend little & build a product before a company) could be the go.
Financial security as a selling point? What percentage of founders do you think achieve financial security as a result of a startup? 80% of small biz's fail in the first 5 years. A big percentage of the rest are "walking wounded" - doing well enough to pay the grocery bills, but certainly not secure.
Startups are high-risk / high-gain bets.
The best way to sell it is, "It's what I want to do. It's what would make me happy."
Presumably you have a relationship where that's important to both parties, right?
Setting an expiration date and a personal investment cap is good, too. i.e. "I want to try this for two years, and then let's re-evaluate. I'd like us to take on no more than $100k in debt, which is considerably less than a mortgage." And, dazzling her with visions of fabulous wealth can't hurt either. Bury her in articles about geeks who have built stoopid little web apps and made zillions of dollars.
Also, don't let the startup take over. Maybe if you're single you can work 14 hour days ... but I've got a family, and it comes before startup (mostly :-). I'd rather be happy than rich.