Rats can be expensive, but no more than any other pet. The initial cost of getting the cage and accessories can be expensive and you have to consider possible vet bills down the road, but in between I wouldn't say they're any more expensive than any other pet. Getting just one rat is not a good idea though. Rats are very social creatures and regardless of the amount of time you think you can spend with them, rats are mostly nocturnal and need companionship during that time and while you're away. Lone rats can be highly destructive and depressed which can lead to health issues. I'd say, look at all the things you need to get started (cage, bedding, food, etc.) and make a list of how much they will cost. If it comes out to something you and your partner feel is feasible, then maybe she will change her mind. If not then getting rats is probably not a good idea at the time.