The Broken Butterfly takes up 8 cells (2x4) in the Attache Case. You can buy it from The Merchant for 38,000 PTAS in Chapter 3: Part 1, or you can find it in a treasure chest by boosting Ashley up through a window. It's in the area right before the hedge maze.
So, if you've passed the hedge maze area, and the are where you hoist Ashley up through a window, I feel you may have missed your opportunity to get that weapon for free. It's been a long time since I've played Resident Evil 4.
You collect money very quickly in the game, though, if I recall correctly. Just kill all the enemies, and collect everything they drop - leave nothing behind and no enemies alive - I ended every game having routed at least 900 total enemies.
And then any weapons you acquired will be in your suitcase when you start a new game, a long with any upgrades you bought before you finished the game.