You could get a couple of female mice for the 15 gallon, I have 3 in a 20 gallon, and they love it. Some basic costs to consider:
Monthly (could be longer)
Litter/bedding (cheaper overall if you get it in bigger packages)
Food: I was surprised by how quickly a bag of food was used up
Toys (can also diy with popsicle sticks, egg cartons, cardboard, etc)
They are really great pets to have, I love my mice so much, every time I change their layout they are so happy and run around everywhere. I also give them a paper towel backing secured with large binder clips, and they love running up and down that thing. They can be a bother at night if you're a light sleeper though.