Most rumors I've heard put it in at 450 dollars for the base version which is probably 16gb, and that would make sense since that's the price of the 5S and would also leave 200 dollars so that the 6 can be sold at 550, (that's probably gonna be the price of the 6s once the 7 comes out)
What I have no clue however is how many storage options are going to be available, I'd guess they're going with just 16 and 64gb at 450 and 550, less likely but I would like to see a 32gb at 500, (The iPod touch does have it and it's 50 bucks over the 16gb one)