dunno where you get your contracts from but in reality an affordable contract nowadays is £25 a month (inc data) and the days of 18 month contracts are well on their way out so would probably be 24months.
so works out about the same at the end of the day.
Mobiles.co.uk et al.
Even £30 a month over 18 months is £540, cheaper than getting sim free and simplicity. Add the cashback and it works out better.
I don't plan on spending more than £550 over the 18 months if I get the Desire HD, it's a pretty standard sort of price if you look around for most high end smartphones.