Hi,
A cheap method is to get some conductive foam of the type used for some ESD sensitive components.
Sandwich a piece of this between to pieces of PCB-material or thin copper (or brass) plate and solder a wire to each. The resistance of the combo will decrease when the two plates are squeezed together.
It might not be as accurately repetitive as force sensors, but you can't beat the price and for initial experiments, it should do fine.