Looking at the source for the scrap box, I found that they produce good loot more often than the junk. There's only 32 items possible, and only dirt is listed twice. Everything else has an equal chance of appearing - meaning that diamond appears 1/32 times.
Looking at the energy costs for recycling, it only costs 280 energy to make a piece of scrap and 2520 energy per scrap box. Since you need 32 boxes on average to get a diamond, that means that for a mere 80k energy you will get a diamond AND 16 pieces of good loot as well. You do need 36 stacks of cobblestone to make this happen.
On the other hand, it takes 9 UU-matter and 9 million energy per diamond with the mass fabber, or 1.5 million energy if you use scrap.
Where this becomes horribly broken is when you have quarries or cobblestone generators.
