Wouldn't the most accurate way of measuring depreciation be using the price of the car brand new (in Yen) and then year of manufacture then compare it to current prices?
If you wanted to, I'm sure someone could come up with an algorithm to include the number of years a certain model was in production (R32 longest out of past 4 models), number of units produced (I'm guessing R32 the most produced but I'm going based on the fact was in production from 1989-1994), etc which would probably tell you that - although the R32 to R34 GTRs were produced in different numbers, the R32s have shown the lowest rate of depreciation despite being the most widely produced.
I guess while you're at it, you could calculate what launch price of those models were in todays dollar terms relative to income for that period and find even more interesting relationships? So you can see to what degree income levels have had on the rate of depreciation for the various 32,33,34 and 35.
I don't know, I didn't do too well at maths nor did I study maths as a degree. But I'm sure one of you guys did. So work it out?