For another comparison, modern KE sabot anti-tank rounds (what tanks shoot at other tanks) use penetrators weighing from 4 to 8kg, with a muzzle velocity of around 1500m/s, giving a low of end of ~9MJ. Granted, this is ignoring air friction, and must punch through the front armor (much much thicker) than the top armor that bombs go through.

Also... as a cost comparison, JDAM (the US GPS guidance) kits cost 20-30k per kit (you strap them onto 'dumb' bombs). For the actual bomb itself, you're talking on the range of a buck or two per pound... the inert training versions are much cheaper than that. Explosives are cheeapp. Silicon is pricey.

See link below for pricing information! (The Mk-84 is the 2000lb 'dumb' bomb, it's listed along with its training/concrete version).

http://www.fas.org/man/dod-101/usaf/docs/munition-cost-11-1....