Web developer to display a value in an input depending on selected radio button

General information for the website: Online Calculator
Kind of development: Customization of existing website
Description of every page/module: The website is built using Wordpress.
The plugin being used to build the calculator is Formidable.

The company sell granite and quart worktops. We have built them a calculator which builds up the prices depending on the users selection.

Process:
- The user first selects the thickness (20mm/30mm).
- Depending on their choice, they will then be shown a separate dropdown for either 20mm 30mm
- They then need to click on an image below to select their material. This then updates the dropdown (The actual select dropdowns will be hidden in the final version).
- The value of the selected material is different depending on weather they want 20mm or 30mm.
Description of requirements/features: What we want:
We want the value of the selected material to be displayed inside the "Rate m2" input depending on if 20mm or 30mm is selected.

If 20mm is selected, get the value from the selected option in the 20mm dropdown and display in the input.
If 30mm is selected, get the value from the selected option in the 30mm dropdown and display in the input.
Extra notes: Please take a look at the website: http://www.ajssltd.co.uk
Username: freestyle
password: newforest123

Once you have access to the homepage, you can then go to: http://www.ajssltd.co.uk/test-image-calculator to see what we have so far.

Please also see attached footer.php to see what I have so far. The commented out part is what I have tried with no luck!