Forgive me but for $2k, I would think that the system has to be much more than just an extra lower return line and valve. The trench is already there so it is just the extra pipe and fittings plus an hour of work at most so it should be less than $500. They probably have something much more elaborate in mind.

A much cheaper alternative is use these returns with a standard return.

I just point one of my returns toward the bottom and it seems to have the same effect. The water gets pretty mixed up anyway so I can't detect any change in water temp with depth and I am not sure lowering the returns would be that much more effective.