Ghostbusters

Ecto is equivalent to points, and as such you rack it up throughout levels by killing the zombies, finding the hidden symbols, breaking stuff, etc.

Whilst 500,000 sounds a lot, in reality this achievement will unlock as you progress - it unlocked for me after the second graveyard level, so only about 3 levels in. However, I was playing co-op and it appears the figure is cumulative. If you want to get this early, play through and at the very end, add another player with a spare controller - that way the ecto the computer has accumulated during the level will count for you, as well as helping level up the other characters. Good luck!!