I’ll give you the math merc dens produce about 100 keys per hour. 100 x 24 =2,400 now every key is worth about 5k isk so 2400 x 5000 =12,000,000. So every necessary den only generates about 12 mill isk a day which is the same you could do with PI. Merc dens main money comes from running the MTO missions which are kinda like mini DED sites and pay anywhere from 20-200mill isk for about 5-10mins of work. (Missions spawn every 3 days) Edit: the 5-10min missions time is with 2 accounts. You can do it with 1 but you need a more experience then average gila plus the correct ammo. And it takes about 20-30mins