Text Attribute with Variable Amount for Gift Card

I am working a simple site that sells special gift cards that can only be used at certain locations but instead of specifying a certain list of choices such as $10, $25 or $50, I want the user to be able to specify the exact amount in a text box. I would essentially want to convert whatever they put in the text box to a dollar amount that would be the base price for the product in the shopping cart. Therefore, someone could even add 15 $12 gift cards to the shopping cart.

Maybe this won't work but is there a way to add another attribute type called "Dollar value" or something that could be configured to be added to the original base price and if so, how would configure it?

I am using Zen Cart 1.3.9e.

Any help would be greatly appreciated. Thanks.