In Australia it breaks down thusly. (for reference average wage is about $80k per annum).

If you buy a vehicle for $50k, you're entitled to claim a tax deduction for that cost, usually spread over a number of years.

However, if you buy a vehicle for $100k, you're only entitled to claim a tax deduction for the first ~$56k (changes each year), unless the vehicle has a large enough carrying capacity that it can be considered to have been designed for the purpose of carrying stuff rather than people.

This rule is designed to disallow deductions for wanky vehicles. Like why should someone be allowed a deduction for driving a wanky mercedes SLK when a cheap and chearful toyota camry can perform the same task of moving a taxpayer from point A to point B. Of course, if someone buys a $300k prime mover (tractor?) designed for hauling 90 tonnes of wheat from a farm to a port, it's just not possible to do that with a toyota camry so you should be entitled to claim the entire cost.

Suppose you have 2 vehicles, both costing $100k, one is a regular sized Toyota truck, and the other is a ridiculous RAM truck or something. Suppose you plan to sell whichever you buy, after 8 years or so, when it's value is $50k.

On the Toyota you can only claim a tax deduction on the $6k difference between the $56k notional purchase price and the $50k sale price, which if your tax rate is about a third then you save yourself $2k in tax, so the vehicle cost you $48k to own for 8 years.

On the RAM you can claim a tax deduction on the entire $50k difference between the $100k purchase price and the $50k sale price. A third of that is ~$16k, so it only cost you $34k to own that vehicle for 8 years.