Not usually that much relative to the cost of the phone. Maybe £100.
This year will be different though as they're throwing a new screen size into the mix and now the 5C exists (essentially the old 5, with a plastic cover instead) .
It used to be that the new model came out, then the top end model from the previous year had a bit of a price drop and became the budget option, but now that there's a 'budget' iPhone already out there in the form of a 5C I'm not sure what they'll do.
Will they keep the 5C in the lineup and drop its price even further (currently £429, although initial launch price was £469), keep the 5S and drop it's price down from £549 to closer the 5Cs current price? Then have the new 6 at £549?
It would be a big step if Apple dropped the price of a new sim free phone to below £400, but if they did it would they pick up market share from the more budget conscious consumer?
I think they'll have a tough time charging any more than £549 for the entry level 6.
It could be the first time they have 3 new phones available to buy instead of 2.
5C at £379 (or maybe £399 if they up the base memory)
5S at £469
6 at £549
They could drop the 5C all together, but I doubt it ,what with all their push on continuity and encouragement of the Apple ecosystem it would kind of make sense for them to have a sub £400 entry level phone to get new customers.