When to breed rats? When you first acquire a snake that refuse to eat FT.
The cost of buying FT from Rodent Pro and raising rats yourself on lab block is pretty much exactly the same. Remember, you must figure in not only the cost of feed, but also the cost of lots and lots of pine bedding.
Labor isn't factored in--you do that for 'free'. And lots of it. You will spend more time caring for rats than you do for the snakes, alas. I have 2 racks full of rats and mice, and I hate them. If I could get every snake I own to eat FT, I would do so. But it's so much easier to start hatchlings on live...so I have to raise the rats. >.<
It's not that rats aren't cute, it's just that they're so horribly messy.