Honestly, i find the best method is what the other guy mentioned. ill just addd that if you grind that for about an hour and look for the best possible deal its easy to get a metric butt ton of cash pretty quick. working at the store helps as well, itll net between 200 - 400 bucks a day. in the very early stages of the game before u have access to the flowers its a good idea to just grind days working