Do yourself a favor, and make sure you get one that is tame. It's no fun having a bird that can't be handled. There are a lot of birds on hoobly.com. I don't know what state you're in though.
Get them where ever they offer a guarantee, just incase it gets sick or dies, so you can replace it.
Make sure when you are picking out a bird, it isn't hiding in a corner, it's feathers aren't ruffled, it's beak is clean, it's rump is not dirty, as these are all signs of a sick bird.
It probably will cost around $20. to $30.00 for a pair of Parakeets, I don't know on the cockatiel as I have never owned one.
Find a local breeder,don't buy them at a pet store.Breeders have much healtier birds and they are cheaper then pet stores.Here parakeets are 8-10.00 and cockatiels are 20.00- 75.00,depending on colors.These are breeder prices not stores.There is always someone near by that breeds tiels and keets.
In N.H.,Tony.
Just putting in that the English Budgies are larger and tend to be friendlier than the typical store 'keets. I hear good things about lineolated parakeets too. Both seem to tame down in just one sitting from what I've experienced with pet store birds. Linnies are hardish to find and pricier though.