Costs on Purchasing a house

Hello

We are going to purchase a house (in Flanders)

Revenue Kadastraal (non-indexed) of the house is EUR 1009 but the actual amount we have to pay every year is EUR 529.69

When calculating the acquisition costs (Frais acte d'acquisition) which value do we have to consider? Is it EUR 1009 or EUR 529.69?

I am asking this question because if we calculate the acquisition costs based on EUR 1009 then we will have to pay 10% tax on the purchase value and if we calculate the costs based on EUR 529.69 then we have to pay only 5% which makes a big difference to the amount we have to pay.

Thank you