Need to create custom variable for product (i.e. custom length of cable)

[piercerides]

I am building a website for a fiberoptic cable manufacturer. They need a product (cable) that a customer can specify a specific length of cable (ft).

Is it possible to do this without creating a huge drop down menu with thousands of options, rather a custom field that when they enter that number is multiplied to the cost factor of the cable (i.e. cable-x would be (700ft. x $2/ft. = $1400)?

Also it is important to note the main issue which is they have a base brice. For example one type of cable has a base price of 1.80/ea with .25/ft.

Otherwise I could simply make the qty the amt. in ft and the cost the multiplier.