Any suggestions for a low cost "dance floor" for a cement basement. Owner is looking for some cushioning under a prefinished floating floor. Something that would give additional cushioning to dancing, something more than a floating floor pad.

Add another layer or two of the foam underlayment. That would be pretty inexpensive. Carpet pad maybe?

Too much cushion sounds like a bad idea for a wood floating floor. Look at Sound Solutions underlayment sold right here.

At one point, some floating laminates said they could be installed over certain types of low pile carpet.

Some still allow it to be installed over level loop commercial directly glued.

I know they do... but it seems to me that too much "spring" would be harmful to a wood floating floors joinery system. A dance floor takes alot of abuse anyway and adding more spring would make matters worse... just me.

Commercial "level loop" carpet is dense enough to provide support. It is anything but squishy.