I would say $200 base and $300 for increased memory. The iPhone 5 will not likely have 4G LTE as this isnt supposed to be ready until around January. The iPhone 5 will likely have the A5 processor currently in the iPad 2, an 8 megapixel camera, and according to some recent leaks the iPhone 5 will have a larger screen and may be thinner. Analysts have talked about a tapered back design, which would look something like the macbook air meaning the back would get slimmer towards the bottom and make it easier to hold.
iOS 5 and iCloud will also be released with the iPhone 5, which will allow wireless syncing, automatic updates for apps, and all iOS 5 devices will stay updated with one another. For example if you take a photo on your iPhone 5 with the new camera app, which lets you take pictures from the lock screen, the photo would appear on your iPad and iPod Touch is they have iOS 5. There are a slew of new features being added through software and my guess is there will be some interesting hardware features as well.