When the price data is read from the database then you need only apply the modifier. I would save the modifiers in a flat file though so you can easily update them later.
This is a fairly simple job though I can not give you exact code because I do not know the variables used in your current script.
You will have to make a form that accepts the currency input,
then set a variable based on the selection
then retrieve the modifier
then apply it to the price.