I've had one cat since midway through law school; he's been the cheap pet, maybe $40/m even when the annual exam and vaccines get factored in (and his adoption fee was only $50).
Adding the dog turned the costs up to 11, though she's well worth it; adding a second cat more than doubled the cat costs (I'm still not sure how, but two cats definitely eat more than twice and require more than twice as much litter as one) and was less worth it (sorry, buddy, but you and I both know it's true ;).
It's hard to imagine life without pets, but it's not too hard to imagine being back in the situation I was in with just the one cat, where I could go away for a long weekend or, honestly, even for a full week and know he'd be fine. Lots of food, lots of water, an extra clean litterbox, and I'd come home to find him mildly annoyed but otherwise chill.
With the dog, I have to factor in boarding costs if I go anywhere, although I can often leave her with my parents and fly out of their local airport instead of mine.
Between the three of them, the costs are about $150/m, higher if I've been taking a series of classes (obedience, agility) with the dog. More than my phone + internet, less than my disability insurance, and, of course, a personal value to me higher than rubies. Except maybe the second cat, who's a menace. It's a good thing he's cute and fuzzy.
