Cheapest/Fastest way to get 12 CR?

[EliEverIsAHero]

For me the cheapest way to get 12 hours was, in a past life, to be a tenured (non-"Other Personnel Services") employee of the State of Florida's government.
This allows you to take 6 credit hours a semester with a tuition waiver from the nearest university. Two semesters and there you have it!

The downside was that your co-workers will mostly be people from Florida.

I concur -- being a staff member for the university (or even for the state/county/city/fed is a good way to go). Alternatively, the graduate assistantship option with full tuition remission + stipend. Lastly, become a hall director for residential life to pay down loans fast.

I can't speak as to local/county, but federal agencies are actually pretty hit or miss with educational benefits. Some agencies are more generous with funding than others, but quite a few are miserly with anything (tuition waivers, external details with other agencies) that doesn't meet immediate business needs. 

DOD may be a good bet, SSA etc. likely less so.