To earn this trophy/achievement, you must possess a total of 1,000,000 Zenny at one time. This is not cumulative, like the trophy/achievement "Commissioned Work" was. Arguably the best way to earn Zenny is by selling off excess monster materials you don't need. Somewhat common, High Rank materials can sell for between 1,000 and 3,000 Zenny each, and it's not uncommon to gain around a dozen or so components per hunt. You'll gain these materials in greater abundance during investigations, and when capturing a monster. The monster need not be terribly strong - even the High Rank Great Jagras's materials are worth a pretty Zenny.
Arguably the best quests to do this on are the High Rank events that task you with hunting several monsters in a locale. These monsters have noticably lower health than normal, and you can end up killing four or five of them in a twenty-to-thirty (generous with the timing here), quest. Or you can just kill the Great Jagras over and over again in under two minutes. Your call. All quests give Zenny rewards, and the aforementioned events give around 30,000 Zenny per completion (assuming no carts), while even a humble Jagras investigation (or similar High Rank monster) should give several thousand Zenny per quest.
While it may be tempting, playing with friends will reduce the amount of Zenny you earn each quest by the number of hunters involved, so only bother with multiplayer quests if the extra hunters are increasing your kill times. It should be noted that material rewards are not appreciably decreased in multiplayer.
Unfortunately, your desire to craft and upgrade the most powerful arms and armor is expensive, and spending is a huge setback towards progress on this trophy/achievement. You can, however, boost it if you're within striking distance. Export your save to online storage or a USB device, then sell all your monster materials... and everything that's worth anything, really. If you hit 1,000,000 Zenny, you'll get your trophy/achievement, and you can then just restore the save you exported earlier. If you have any realistic plans of hitting Hunter Rank 100 (much less going for Mini Crowns and Giant Crowns), however, you shouldn't need to do this, as the grinding it'll take to reach those lofty goals should get you the Zenny - or the materials to sell for the Zenny - that you need.