If you were to pay for a year, you’d need to earn 50 million ISK daily for a year to be able to PLEX your account annually when CCP has their Omega NES sale.
Multiply this by every account you want to PLEX. And if some are trained up where you need them to be, you can make around 1.2 billion ISK every month just extracting skills. Not enough to completely offset the monthly cost - but it will cover 80-85% of it.