Well... some standard sizes for you to look up would be the 90, 110 XH, 120, 125. Prices of used tanks depend largely on your location. Down here, we get em cheap on Craigslist, but a lot of other areas are more expensive. The prices for new tanks are pretty standard with the exception of sales. What you see at your local pet store is pretty much what they cost everywhere.
A lot of times it pays off to be patient and wait for the right deal on a used tank. HN1 and I got a 4'x2'x2' 120 gal
with stand for $150, and a 60"x18" 120 gal
with stand for $165. Both are older, but in good condition, and IMO
a lot of the older tanks are built much better than a lot of newer ones. For example, the 60x18 120 gal
is an old Oceanic with a glass center brace. I've got a 75 gal
Perfecto that is so old, and made of such thick glass, that it doesn't even have a center brace. They don't make em like they used to. :p
Here is a link for some standard dimensions that you might find while you are waiting for the right deal to come along. Aqueon » Aquariums | Products
*edit* this is by no means an exhaustive list.