Yeah but when your startup isn't covering the bills and you're already eating and living cheap - what do you do?
So when there isn't money in the bank you go out and you MAKE SOME DAMN MONEY.
If all else fails get an out-of-focus purely-for-money project. There's usually a lot of those lying about if you're willing to work on them.
full disclosure: my startup has yet to pay bills, but we have a knack for getting government grants
You should do what you love. Make peoples' lives easier. Financial gain is just a by-product.
Go out and change the world, with a passion to change the world (rather than financial gain), and the rewards (whatever they may be, money or otherwise) will come.