Price RoundUp/RoundDown - CultureInfo is applied but is wrong

ok, ill try to explain this the best i can, i hope you will understand.

the primary Currency on my site is HUF (Hungarian Forint),
in hungary, there are no more small coins (1,2) even due they once existed.

the fixed tax (AFA) is set for 27% (comment: AFA is like VAT in the US)

now, lets say a product has the base price of 124990 HUF,
after calculating tax, we get 158737.30 HUF

there are two problems with this price.
the first, HUF does not have decimal places anymore.
that was kind of easy to solve:
TaxService.cs
        protected decimal CalculatePrice(decimal price, decimal percent, bool increase)
        {
            decimal result = decimal.Zero;
            if (percent == decimal.Zero)
                return price;

            if (increase)
            {
                result = price * (1 + percent / 100);
            }
            else
            {
                result = price - (price) / (100 + percent) * percent;
            }
            return Math.Round(result,0);
        }
so now the price is rounded to 158737.00 HUF
but, i still have two problems with that.
1. the decimal places should not be seen at all (since they actually do not exist)
2. since HUF does not have 1/2 coins, the price should be rounded up to the nearest 5
example: 158737.00 HUF should be actually 158740 HUF (no decimal, rounded to the closest 5)

can anyone help me with this? how can i acheive that?