Transgenes can affect the fertility of mice or mess with their sexual behavior, it's not uncommon for transgenic lines to have erratic breeding schedules and re

Transgenes can affect the fertility of mice or mess with their sexual behavior, it's not uncommon for transgenic lines to have erratic breeding schedules and reduced litter sizes compared to wild-type. Then if you get a litter, maternal care is very important. If a transgene messes up the mother's ability/drive to feed the pups, then they aren't going to make it very long (though you can foster/wet nurse them). Finally, depending on the required genetic profile, you may have a situation where only 50% or less of the offspring have the transgene of interest. Some of this is "worst case scenario" stuff that wont apply to most lines. The real reason for the high price is mostly 1) that its expensive to make the transgenic mice in the first place and 2) demand is high enough that they can get away with charging that much.