Have a base price then a price per Unit on "Fractional and Fixed quantities" module.
Allow us to set a base price for the product and then add a certain amount per "unit" in the "Fractional and Fixed quantities" module. Example. It cost $10 as a base price for a custom cable and it's $1 per foot. So a 10 foot cable would be $20 but a 5 foot cable would be $15.