I paid $125 for my silver TT C-Mel in perfect condition. Although Music Medic has a pad kit price for most horns starting at about $50, I didn't see one for the C-Mel in the pad configuration I wanted. Ordering individually, the pads came to about $85. Unfortunately, some are on back order until the end of May. Until then, I can only play two notes. Wahhhh.
I have the Music Medic repair kit for clarinet, so I already had extra corks, springs, felt, teflon, and the necessary tools. I think that the kit is also about $85 and, if you order the sax kit, you'll get some extra sax pads. MM may also have repair instruction books. If not, they're available. Figure $200 for completely rebuilding a horn yourself that has no major problems. More than twice that much if you take it to a shop. Once you see how much time and tuning is involved, you'll realize that the tech earns his fee.
It's possible to get a horn that only needs a couple pads, a couple corks, maybe a spring or two, but if you're buying off Ebay, figure it needs everything, bid accordingly, and it may work out alright. Unless the horn says it's rebuilt by somebody experienced and you can verify it, statements like "pads look okay, but I don't play" mean nothing except buyer beware. I'd say $200 is tops for an Ebay horn that looks to be in really good shape. If you pay more, or intend to have it rebuilt at a shop, you'll have more into it than you could get for it. On the otherhand, you'll likely have a nice C-Mel.