Yikes, that is expensive...
What if you just spaced the neuters out over several months? I enjoyed having a mixed colony, and never had to worry about accidents.
I know, I know, everyone feels it won't happen to them, but even the most experienced people on the Shack have ended up with an oops litter.
The other thing to take into consideration is that the boys and girls will be in a constant state of stress, as the girls cycle into heat and can smell the boys, and the boys will be stressing because they can smell the girls, ready and waiting. Sex is a natural drive, and there's only one way to eliminate that for the ratties...