any number crunchers out there

[kfrimr]

I am trying to figure out from purely financial numbers what is the best time to trade up our DRV? Considering the deprecation of the current 5th and the increasing cost of the new coaches is there a best time, strictly financially? Does the bottom fall out on the value of an older model or does it continue at the same pace of depreciation? From the best I can tell last year NADA value dropped 3 thousand dollars and I just looked and this year it is 3 also. So does that mean that next year it will be 3 as well? I guess what I am asking is at what age is the greatest drop in value?
Kent