Which is cheaper - new or 3-year old over 8 years?

You also need to determine length of ownership. If 20 years I'm guessing new, if 2 years then the 8 year old one.

Again what is the length of ownership?

On my class A I determine how long I wanted to keep it (five years) and the amount of money I was willing to lose on that investment. I took what I was willing to lose and doubled that amount. That was my buy in point. Last summer I sold it after six years with a 35% lost on investment. I feel pretty good now that it's all done and over with.