If you purchased the property, the purchase price is what you paid for it.
You’ll find this in your purchase contract. 

If you built the property, the acquisition value is the total cost of the plot and the building(s) at the time of construction. These costs are documented in the invoices related to the construction. 

If you’re selling a share in a housing company or jointly owned property, you must base the calculation on the purchase price and add the apartment’s share of joint debt, then subtract the share of joint wealth as at 31 December in the year before the purchase year. You can request these figures from the housing cooperative/building society or use the figures from the original purchase documentation.

If you’re selling a property that’s been owned since 1990 or earlier, you may adjust the purchase price/acquisition value upwards, which may reduce your tax.