To go along with everyone else the cost is only part of it. Time is a big part of it as well. Granted for those few snakes it won't take much time to care for the rats but it is added time. Also the smell is another issue to deal with. The more often you change the bedding the less smell you will have to worry about. I started with 4 balls and was breeding rats just to feed my 4 balls and I was actually spending more to breed than buy frozen. I expanded my breeding and have started to sell to a couple friends which has offset the cost and it's actually slightly cheaper now to breed than buy. So think about that as well. So look at cost, time, cleanliness, and then also the joy/headache you might have.

P.S. I'd suggest a rack instead of tanks. just my $0.02