It's difficult to claim that a higher depreciation results in money lost over the long term, unless the initial outlay is identical.
Let me illustrate with a concocted example.
Tractor A cost $30k, and over a period of 10 years, it will lose 40% of its value, so it will be worth $18,000.
Tractor B cost $25k, and over a period of 10 years, it will lose 50% of its value, so it will be worth $12,500.
At the end of the 10 years, you look at this and say "Tractor A is worth 5500 more than than tractor B, and only cost 5000 more, so I came out $500 ahead!"
However, if you had put that initial "extra" $5000 saved in a simple savings account that returns 1%, you'd actually have $5526 in that account after 10 years, or you would've been $26 ahead by getting tractor B. Obviously, if you were actually going to invest that $5k, you should be able to manage more than 1%.
Or, since you're on TBN, you would've been able to buy $5k more attachments for tractor B, which after 10 years are probably still worth a decent chunk and you would've been able to get a lot more work done.