I think having a white mouse is very economical. It is really tiny and eats everything that comes its way. We do not have to specifically feed it. I do think that most of the smaller parts are very economical, also they should require very less maintenance. Most of the birds and fishes are really economical, but do take a lot of time and effort for maintenance. Also, they do not last for a very long time. I had a bird which lasted for two years and most of the others died really soon.
I don't think I would ever have a mouse, it's simply not my thing. I believe that soon enough I will get a small turtle though because my kids are really eager for a pet and a turtle is something both economic and that doesn't occupy much space, so it really meets my requirements. I think Santa is bringing a turtle this year.