The Binding of Isaac's large-scale expansion pack is coming in late May for the price of $3, according to Team Meat. It will contain 50 percent more content in all, so that seems like a pretty good deal for the price.
A NeoGAF thread (via Eurogamer) compiles information about the expansion from various tweets of creator Edmund McMillen. In all the expansion will include more than 10 bosses, 100 items, 20 enemies, five ultra-rare pick-ups, 10 new challenges, and a new character, among other additions. That certainly fits the bill as a so-called mega expansion.