With 5-7 accounts ratting in null I doubt you'll get more than 200m isk/hr. Plex prices are through the roof right now, so id wager you'd have to rat about 30 hours a month just to plex. May as well get a second job and spend that money on subs at that rate.

WHs and Incursions will get you more isk, but of course there is a much larger barrier of entry, not to mention initial investment.