You could always perform calculations on the printer template, for example:
[= Format (ToNumber (ToNumber ('{PROP PRICE}') * (ToNumber ('{QUANTITY}'))))]
This will multiply the modifier price by the line quantity ordered, also:
[= Format (ToNumber (ToNumber ('{PRICE}') * (ToNumber ('{QUANTITY}'))))]
This will multiply the line item by quantity, you could also use:
[= Format (ToNumber (ToNumber ('{PROP PRICE}') * (ToNumber ('{PROP QUANTITY}'))))]
This will multiply the modifier price by the quantity of modifier added.
This should be able to account for most circumstances and combinations of modifiers/quantities and this is what I use on my bills and receipts