I found an old damage chart. A 500 lb bomb does 562 "pounds" of damage. A 5" HVAR does 156 "pounds" of damage. So according to this chart, 3.6 HVAR's would be equal to a 500 lb bomb, assuming all the rockets hit the same spot.
In real life (and in game), the 5" HVAR was roughly equivalent to a 5" naval shell. In fact, the first 5" HVAR's were literally made from 5" HE shells attached to rockets.