I need to create a product modifier that will calculate a price modification based on some outside factors. To do this I thought I would first start with a product modifier that just has the user type in the amount to modify the product, but I don't see how this can easily be done.

How do you make a product modifier with a type-in price modification amount?

That is sorta what I have in mind. The first step I want to accomplish is to add a modifier to a product that allows the customer to enter in an amount to be added to the cost of the product.

For example, lets say the product was a Boat Excursion and the customer could choose an arbitrary $$ amount for "Entertainment Package". Where by pre-paying for a specific amount they get a better value or whatever.

The trick is I need to allow a user to enter in a $amount and it will be validated to be sure it is > 0 and then is ADDED to the item total in the cart.

I assume this will need to be a custom control, but I wanted to know if there was anything out there that could do it, or even something I could use as a sample before I start fumbling around to make my own.