The prices be tiered similar to this:
White Iphone 4 32gb
Black Iphone 32gb/White iPhone 16gb
Black Iphone 16gb
Apple is (reportedly) still going to be selling the 4 when the next iPhone comes out, which will help it increase its worth. My guess is that the White 32gb will go for around $450, the Black 32 and white 16 around $380-400ish, and the black 16 for around $350. Thats a dip in price of about $100-$50 of what they are selling now. This is just a complete guess though and is determinate on the demand for the next iPhone, the more people want the "5" the more people that will be selling the 4, the less there will be a demand for the 4, the less it will sell for.
Honestly though, I have a white 16gb, and my girlfriend has a black 32gb, and if can I get $350 for each one when the next iPhone comes out, I'll be happy because we'll get the next gen for free (assuming theres a model for $299 of course)
Edit: this is going off the current price list for iPhone 4's in Southern California. White 32gb is in the $500's, black 32gb and white 16gb is $450-$500 and black 16gb is $400-$450