basically the cheapest you'll find will probably be the Original Rebel 300d (or nikon's equivalent) the 350d from Canon I've seen go in the $250 range.
yes, that is signified by the d at the end of the numbers. The XSI is 3 generations newer than the 300d (otherwise just called the digital rebel I think)
It's old technology, but it is an SLR, and it will work.
Some prices may seem like highway robbery, but if you research how much a camera went for new you might think again. Ebay is your best bet. There is no telling how much someone is going to sell a used camera for. There could be a $100 difference between 2 identical models. If prices are too high then you need to wait til one becomes available in your price range. Buy a used camera is like buying a used car. There are many things that effect its resale value.