function calculateTotalCost(uint bannerId) internal view returns (uint) {
        
        uint totalSecondsPassed = block.timestamp - banners[bannerId].timestampTaken;
        uint totalCost = totalSecondsPassed * (banners[bannerId].bidAmountPerDay / 1 days);

        
        if (totalCost > banners[bannerId].balance)
            totalCost = banners[bannerId].balance;

        return totalCost;
    }