This might be helpful...
https://forums.macrumors.com/threads/749431/
Also, does it matter to you how much depreciation occurs or are you really literally asking about the price after 6 mos? The data in that thread generally bears out the idea that depreciation is approximately proportional to value -- the 15" will be worth more than the 13" after a given amount of time, but it will also
lose more value in the given amount of time.