First, reduce everything to ISK per dollar.
PLEX = 76.8
Daily Skill Injectors = 96.6
Small / Large injectors you cannot buy directly from the NEX store; you can only buy (empty) Skill Extractors, the points are sucked out of existing characters’ heads, by players who are willing to do that. So because you cannot buy directly from the NEX, you ultimately have to use PLEX as a converter, so assume the PLEX conversion rate.
Skill Extractors = ?
You can look at other New Eden Store (NEX Store) items; maybe there are popular SKINs (ship paint schemes) or clothing items that are popular that may convert with better rates, but be careful because these things may take a long time to sell on the market.
Short term, I’d say get a couple of the PLEX or Daily Injector deals. 1.5 B ISK should be sufficient for some time if you don’t fly bling PVP ships. Part of your savings will have to come from not flying what you can’t afford to lose, as much as possible.
Long term, as explained above, the cheapest deal is to create a Skill Farm Alt. Create a new (free) alpha account right now, and create a character and queue up all the Alpha skills. This will train the initial 5 million skill points for free. Keep the account alpha (free), until…
CCP is likely to offer a 40% discount to brand new Alphas to go Omega for a year, comes to about $8 per month if you buy the whole year. Check periodically for that deal, and take it when they offer it. Once you take it, your Skill Farm Alt will go Omega, train about 1.6 M skill points per month, with a long queue that you don’t need to babysit.
Then, every month, you extract the 1.6 M skillpoints into 3 Skill Injectors. Subtract the cost of the 3 Skill Extractors that you need to use each month, and you should end up with more ISK/dollar than the PLEX. The whole thing relies on the fact that subscription with that discount is much cheaper than 500 PLEX to go Omega per month. And the Skill Injectors that you get from the alt are very desirable and will sell instantly.