Hi Jimmy, I'd advise maybe phoning some camera stores which you know sell 2nd hand gear and also viewing previous auctions on sites like ebays to see what people are paying for them.
You can also find out the new price of almost any product through the co-branded site we've built with Pricegrabber. Just click on:
Then search for the product you're interested in for an idea of its new price.
One word of warning though: as I mentioned in the other thread, your XTi will effectively multiply those lens focal lengths by 1.6x, so the 35-80mm you mention (which I also don't believe has USM) wouldn't be a very practical general purpose lens for your body.
If you're after a low cost general purpose zoom to get you started, you can't beat the Canon EF-S 18-55mm kit lens, and if you can't find one on sale, I'd seriously look into seeing if you can return your body and swap it for the XTi kit which comes with this lens for little extra money. You can't buy a cheaper general purpose lens for the XTi.
If you have a higher budget though, the Sigma 18-200mm is a good option, as is Canon's own EF-S 17-85mm which includes Image Stabilisation. Both are reviewed in our lens section.