If you genuinely own the gold for investment, don't rely on its original purchase price when calculating Zakat.
Key points
Keep a statement detailing its weight, purity, the nature of your ownership, and its value at the time of calculation, and separate any storage fees from the gold itself.
The final Zakat calculation depends on your individual circumstances and total assets; therefore, my bank provides the required information but does not issue a personal fatwa.
The figures, fees and limits in this answer vary by bank and change over time — confirm them with your bank or the official source before acting.