It really depends on the size of your collection (as bmm has already said), and how much you value your time.
For instance, I have approximately fifty snakes (would you believe no corns?), and I expect that number to jump dramatically at various times during the year.
While it may seem more economical for me to breed my own rats and mice, time is a precious commodity for me. For now, I feel that it's cheaper for me (after you factor in the time involved) to order in bulk rather than maintain my own rodent colonies.