If you're buying it from your provider:
Log in to your online account (it's easy to setup if you don't already have one) and look for a link that says something about upgrading phones (you may have to navigate a bit to find it). That page will list all of the phones you're approved for and their cost. Alternatively, just call customer service. I'm with T-Mobile and they're extraordinarily helpful with stuff like this and won't try to push you to buy if you're not ready.
If you're getting it from another place such as eBay:
Just do a search. The Curve is an older phone, so you can get a brand new one with accessories for roughly $150-$200 off of eBay (I just did a quick search). Only caution is that if you're getting it off of eBay, make sure it doesn't require a contract because many do - it will clearly state it if it does.
