It might just need to be cleaned. If it's a membrane keyboard, try taking it apart and cleaning the circuit-board contact for the minus key with a pencil eraser. Most keyboards are not terribly mechanically complex, so that shouldn't require a degree in engineering.

I've also had success in the past with cleaning keyboards by putting them in the dishwasher. Just make sure you don't have the heated dry cycle on.

You could also get a USB numeric keypad, like this cheap one or this, probably higher quality, one.

When I was finding those products. I came across this. Very clever.