In the UK, 505 pounds (16Gb) plus another 15 to have it legally unlocked via the O2 website. That takes up to 14 days!
But if you buy it straight from the Apple website it's factory-unlocked rather than jailbroken and can be used with any provider; just ordered one myself - GB£510.00 all in including delivery (to a UK address for onward transport to NBO). Only downside is that the iPhone 4, like the iPad, requires a Micro-SIM but I'm given to understand that you can create this yourself with either a SIM-cutter or a pair of scissors and a steady hand. Dunno if this would work for you. I guess that the UK shops might offer the same deal although I don't know for sure?
Edited to add: Just found this info on buying SIM-free from a UK retail store
Apple Retail Store - iPhone at the Apple Store
HTH