If O2 and other carriers offer an unlocking service from day one, either with a fee or without, all this talk of the difference between SIM free and PAYG is irrelevant. Remember, when the other iPhones were launched it was not possible to get an official, factory unlocked iPhone in the UK. The result was that companies like play.com sourced iPhones from other countries, like Italy, that did sell factory unlocked iPhones, and sold them at huge premium. As soon as O2 offered an unlocking service no-one needed to buy from play.com anymore.
I don't believe the price for PAYG and SIM free is different. The SIM free models were simply sourced from outside of the UK, and so had extra costs in sourcing and importing them, plus the third party profit margin. Here in Saudi, all iPhones are sold by the official provider factory unlocked, the top model has always cost SR2999, which is about £545 at todays exchange. You just go to the shop, buy the phone, use the SIM provided or bung any other one you want in. So you can see, the prices are comparable in other parts of the world that sell factory unlocked iPhones from official providers.