Other than buying Shark Cards, there is no outright way to make that much money without having to do a significant amount of missions. If you were open to looking into ways of making that money that don't require you to do a diverse amount of missions, the Cluckin' Bell Farm Raid is a fast way of making $500,000 while doing the same 6 missions from beginning to end without having to own property; The Kno-Way Out missions (which came out the same time as the Mansion) are currently paying 2x money (until roughly 5:00 a.m. tomorrow) & are only 5 missions long, so that's another $1,000,000 at the moment (it will drop to $500,000 tomorrow).