I am working with a dance group who use a temporary stage that can be easily set up and broken down for shows. It stands between 2 and 3 feet off the ground. The only problem with this stage is that it is hollow underneath, so heavy footsteps and jumps, which are somewhat common in these dances, create a lot of noise. Are there any cheap solutions for baffling the underside of this stage to prevent this noise?
Are you sure echo is the main problem? It shouldn't echo much unless it's on a hard floor. Setting it up on top of something soft like carpet remnants should pretty much eliminate echo under the stage. But even if there is some echo under the stage, it's unlikely to be the loudest sound you're hearing.
Maybe you're hearing the clattering noise that happens when the stage's joints are compressed and different parts clap together. This type of sound is going to be a lot louder than echos under the stage. You may need to identify which joints are to blame and stuff bits of felt in them.
One more suggestion is to check your dancers' shoes. Hard soles landing on a hard stage are going to make noise. Padding the stage and/or the shoes could help. Just make sure they'll still have the traction they need to dance safely.
I would suggest carpet padding. My local home center has 270 sqft. rolls for around $100 U.S. I would lay it on the floor prior to assembling the stage. It will be reasonably light and can be rerolled for easy transport. It comes in a variety of material. I would select a material that looks like it will survive repeated handling. Any tears that do occur can be quickly repaired with duct tape.