It's a great question!!! Let it simmer overnight and the activity will pick up. Very interesting to see how it's perceived by each poster. I don't think any posters, so far, have tried to get an accurate number for you. Would take awhile in the recliner with stubby pencil and calculator to get much accuracy. A LOT of variables.
One that hasn't been mentioned yet. How far back are you going?? Is this your first tractor?? If not do you count the initial cost of the first, and every trade since?? Otherwise you wouldn't know what your tractor cost. Hmmmmm......