How do I make a price per unit fill in the blank?

I want to allow users to fill in the price with a suggested minimum.  I'd like a minimum, but from what I've read, that is not an option.  We are doing multiple quantities, but would like them to donate more than suggested if they'd like.

https://form.jotform.com/61195271689163

You can use calculation values:

http://www.jotform.com/help/301-How-to-Assign-Calculation-Value

And total everything up in a calculation field:

http://www.jotform.com/help/259-How-to-perform-calculation-in-the-form

Then send that total to your payment field:

http://www.jotform.com/help/275-How-to-pass-a-calculation-to-a-payment-field

Create a radio button field with the values you'd like to present and assign those calculation values.  Then, add a number field to the form for custom amounts.  Add those up in the calculation widget and use that total to your payment field.