Much less than your estimate, actually, since everything was apparently crushed by the bulldozer before being covered up with cement (according to news reports of the time). Also, the stuff buried included "parts", which could easily have included millions of raw ROMs... very compact.
I still dont get why no one has actually produced "something" from this burial site...you would figure if it's there Geraldo or someone would have been digging it up by now lol.