You will need to gather the yellow Moon Crystals in order to purchase Upgrades for Mondo. This achievement will take longer and be more tedious than it's right-hand counterpart, however. With enough grinding you should be able to obtain this early on in your third play through. Alternatively, for $1,000,000 for a set of 100, you will be able to purchase your upgrades if you get tired of grinding.
Money Grinding Method:
The easiest method (that I've found thus far) of accumulating money quickly is to set the game to Nightmare Mode and replay Episode 1. The Episode is 35-38 seconds long and will reward you with anywhere from $720,000-$729,000 money per play through on Nightmare Mode. REMEMBER TO UNEQUIP THE DARK BOOSTER AND ULTI-MONDO COSTUMES WHEN MONEY GRINDING.