plex cost just under 800m at the moment. If you have enough accounts to run incursions then thats a good way to earn isk enough to plex, next best would be wormhole sites where you dont need as many characters but the isk flow is less stable. For incursions i believe the minimum you should have is 8 dps, 2 logi and 1 link character with 12 dps being the best. For WH sites you can do very well with 5-6 dps 2 logi and 1 link for c5 sites, for c3 sites you dont need links but theres less isk to be made.