DF 4.10.4.27125 / DNN 7.0.3

A form has:
* Radio button group named Location with options: A, B, C
* Text field named Payment Amount (PaymentAmt).

If the user selects option C, then the form needs to validate that the payment amount it greater than $100.

If they select A or B the payment amount can be any value.

How would I put this together? Can you point me in the right direction?

Thanks.