You could always see what the retention lot will offer you on a phone u want, its what i often do with t-mobile, as over the past 5 years iv had the same contract, but tweaked so much to how i like it for peanuts basically, so i generally try and get money off phones i want.
Last time i upgraded nearly year and a half ago, i wanted the HTC HD2, but on my monthly contract they wanted £250 of me for the phone, so i plugged away, and managed to get that cost down to £90, so still had to pay for the phone granted, but no where near as much as a new customer in my contract price bracket (£10-15).
Im due for an upgrade this Dec, so im planning on trying my cheek again, and gonna see if i can get the sensation or S II for far far less money lol.
Basically if you don't try, you will never know, worse they can say is, sorry sir we can't do that.