How much does a used iPhone 4 without contract cost?

My friend is offering to sell me his old iphone from verizon and we haven't come up with a price yet.
ALSO:
me and my friend both have verizon and i have a crappy flip phone but i was hoping to upgrade to the new iphone. If i buy my friends phone, what exactly do i need to do to connect it to my plan? Is it a complicated process? I'm not looking to renew my 2 year contract, although it already expired.

$349

The IP4 will be discontinued in a matter of months, as 3 years antique already.
When the IP5S is released the IP4 is history ( IP5 IN IP3GS OUT)

The cache drops just like the price will, who invests in a Really OLD phone after you
New phones have WARRANTIES (A cynic would say your friend is jumping ship )

All iPhones are overpriced and you can easily get a phone that is better for less, this is a phone that is
in another year at 4, is 2 phone contract ago and you want to buy it… Don't you will regret it and to clarify pre owned is NOT NEW.

http://www.itproportal.com/2012/09/06/8gb-iphone-4s-version-be-introduced-iphone-5-launch/