The expenses associated with animal husbandry at most research institutes raise the price to ~$1/day per cage of 5 mice.
So, let's say you need 20 female mice for an experiment (e.g. 4 groups of 5). That means that you need to have >40 offspring in order to get enough female offspring. That's at least 7 breeding pairs (assuming you're not harem breeding) and you've got to maintain them for 6-8 weeks until you can use them. That's >$1,000 in breeding costs before you get to the actual experiments.