Assuming you’re using +5 implants, have remapped your attributes to 32 and 26 for a primary and secondary attribute and are training a skill, which benefits from the remapping will you get 45 SP per minute:
32 + 0.5 * 26 = 45 SP/min
This will produce close to 2m SP per month:
45 * 60 * 24 * 30 = 1,944,000 SP
At the current price for a Large Skill Injector (713m ISKs) and the highest amount of SP available for an injection (500,000 SP) does the amount of SP equal around 2.8b ISKs:
1,944,000 / 500,000 * 713,000,000 = 2,772,144,000 ISKs
On the other side of your bill do you have the cost for a PLEX (3.34m ISKs) and the cost for the skill extractors. So do 30 days of Omega game time (500 PLEX) cost around 1.7b ISKs:
3,340,000 * 500 = 1,670,000,000 ISKs
The cost for the Skill Extractors (375m ISKs) to extract a month of skillpoints costs around 1.5b ISKs:
1,944,000 / 500,000 * 375,000,000 = 1,458,000,000 ISKs
In total do you need to spend around 3.1b ISKs to make 2.8b ISKs:
2,772,144,000 - 3,128,000,000 = -355,856,000 ISKs
The difference will likely find an explanation in the different cost for a month of Omega time and a stack of PLEX at the CCP store. A year of game time costs £89.99 and is equivalent in time to 6000 PLEX, while a stack of 2860 PLEX costs £84.99, which makes PLEX about twice as expensive:
(6000 / 89.99) / (2860 / 84.99) = 1.981339
A player, who farms ISKs by buying Omega game time directly from CCP and doesn’t first buy PLEX, will get a better deal. So would £100 be worth 11.2b ISKs when bought through PLEX and be worth 17.5b ISKs when bought through Omega game time:
100.00 / 84.99 * 2,860 * 3,340,000 = 11,239,439,934.11 ISKs
versus
100.00 / 89.99 * 518,400 * 45 / 500,000 * (713,000,000 - 375,000,000) = 17,523,867,096.34 ISKs
(518,400
is the number of minutes in a year - or more precisely 12 months of 30 days each.)