Home Gym Flooring

Im starting a little home gym in my garage and i wanna put like a rubber or foam flooring. but its all super expensive does anyone know somethin cheaper to buy or where?

I want to set my home gym equipment (treadmill, bike, bench, yoga mat) in a shed with a gravel floor. As this is just for the summer, I do not want to invest too much in a floor to protect my equipment from scratches and the treadmill motor from dust. I figure about 16′x16′ would do. Any suggestions besides plywood?
Thanks!