I believe you are conflating cost with adjustable value....

I believe you are conflating cost with adjustable value. Adjustable is used to determine the balancing adjustment on the taxable use portion of the asset and cost is used to determine the capital gain on the private use portion of the asset. If you look at the substance of the transaction and split it into a private use asset and a taxable asset you can work through the logic of what is occurring for depreciation and capital gains..

10% of $5,000 = $500 cost
10% of $7,000 = $700 proceeds
$700 - $500 = $200 capital gain.

90% x $5,000 = $4,500 cost
10% of $7,000 - $6,300 proceeds.
90% x $2,000 = $1,800 deductions claimed.
$4,500 - $1,800 = $2,700 adjustable value.
$6,300 - $2,700 = $3,600 balancing adjustment.

I do note that the above formulas do not strictly follow the exact formulas in the legislation (though in this instance they do result in the same outcome) and are simplified for illustrative purposes.

With that being said if you have a specific question about your specific circumstances you should discuss this with your accountant or other suitably qualified professional.