I have 3 lines. This year, only 1 was available at full upgrade and the other 2 were subsidized. So heres how it went down...
A (me) - had an 8gb iphone 3g
B (gf) - had a 16gb iphone 3gs
C (mom) - had an 8gb 3g
I upgraded all 3 lines and laid out the $$. (A) $199 + (B) $299 + (C) $299. Came to $955 (give or take with tax and that $30 fee they charge)
I sold both (A and C) 3g's for $150 each, that was $300 cash. I also sold (C) iphone 4 brand new in the box for $750 cash. I gave the (B) 3gs to my mom.
When I was done, me and the gf both got 4's and my mom got a 3gs and we spent nothing.
I do that kind of stuff every year to get iphones. BUT, the first year when the first one came out, I laid out $1200 to get 2 brand new phones for me and my gf. Since then I've been hustling my way into each one for nothing or close to it.