I was trying to save up for the Kosatka submarine to run the Cayo Perico heist solo, but getting griefed in public lobbies made earning impossible. I eventually gave up on the "legit" way and went to https://playhub.com/gta-5/money to just purchase the sub outright. Now I can actually make money in the game on my own terms, but that initial boost was essential to getting my foot in the door.