Saving 9 engines from a Falcon 9 is a considerable savings but saving all 27 engines from a Falcon Heavy launch would bring the cost per kg down to perhaps $100.
Slashdot is filled (at least theoretically) with smart people... so why do I have to keep explaining this?
It's virtually impossible to determine how much recovery will bring down the costs of launch because we don't know how much it will cost to refurbish the recovered vehicle. Certainly it will be cheaper than building a new one, but how much cheaper is impossible to predict... especially in the beginning with zero experience.
The case with the Shuttle is instructive... it took dozens of flights to go from removing and dismantling the SSME's after every flight to only removing them for inspection every third to fifth flight and only dismantling them for cause. (And they were on the second or third block of engines by the time they reached that point.)
Falcon has the additional problem of figuring out how to inspect and re-certify the tank.