As someone who is considering submitting microstock, I have a couple of questions I hope someone could answer.

First, how large of a portfolio of "stock-worthy" images would you recommend before making your first submission?

Second, how do you make this cost-effective? I definitely don't plan to make stock my day job, but it doesn't seem that shooting photos purely for submission to stock agencies is a good use of money. I say that because it seems the best selling images are creative, and I would have to pay a model to make those shots. At the going rates for microstock, it would take quite a while to recover that expense.