[kyleb]

I have three cats and I love them.

i) Not much if you adopt one with initial shots and vet checkups done (usually around $100-150 for a kitten with most of the shots done, usually lacking rabies shot which costs $50 or so at the vet). For my three cats I pay about $60/month in food and litter plus the occasional treats, toys, and a ballin' water fountain for them to drink out of ($40 for the fountain, $10 for new filters every 2 months). Pretty cheap.
ii) They are pretty well potty trained if you adopt from a shelter and they are spayed/neutered early. As for destroying your furniture, they're gonna do it no matter what. You can declaw them, but most people think it's "inhumane" to do so. My cats just [censored] up my cheap couch from Ikea, so I don't really care.
iii) I wouldn't leave them alone for a few days, but "kitty sitter" services aren't expensive.

The best toys the cats will love are:

-A jar of catnip that you can put on their scratch posts once a week or so
-A laser pointer

That's all you really need. Getting two kittens is usually best so they have someone to play with when you are out/busy. Get males if you can.