Note-this is a rough calculation. I am not in the business of giving tax advice.
Sale price €400,000
Purchase price €50,000
Indexed at 1.503 €75,150
Expenses of sale and purchase (say) €10,000
Gain on disposal €314,850
Annual exemption €1,270
Taxable gain €313,580
Tax payable @ 20% €62,716
Any enhancement expenditure may also be taken into account, subject to certain restrictions. I wouldn't be 100% on indexation anymore. Where there is a signifcant amount of money involved, it may be worth your while getting professional assistance.