Yeah, keep tropical aquarium can get expensive especially on the initial investment. Once everything is set up, the maintenance is not expensive at all. In fact, not much maintenance is needed besides one a week partial water change and once a day feeding.
Would they against you for owning a 5 gallon or 20L Betta tank in your own room? I'd think owning a single Betta is a good start for going for bigger more complex tropical community aquarium.