THHN versus NM for conduit wiring?
I pay $48-52 per 500ft for #12 THHN/THWN-2.   I do not use #14 at all. 

My preferred supplier is City Electric, and I shop around other proper electrical supplies, and Menards on 11% off days. 

I do not buy ground wire because I use EMT properly, I use enough conduit clamps to hold it where it belongs, and I refrain from routing it unguarded in places at risk for damage or pullout. 

When I find an odd situation where I worry about EMT parting, I dive into my junk drawer and find some old solid THHN or Romex that I had pulled out earlier, and strip down a bare wire of enough length to link the two junction boxes on either side of the danger zone. 

By the way, I use stranded THHN/THWN-2.  After the pleasure of working with stranded THHN/THWN in conduit, the idea of choking NM down a conduit is laughable. 



You cannot strip the sheath off NM as a "cheap" way to get THHN.  The reason is obvious once you work with the various wire types, but NM wire insulation doesn't even begin to compare to THHN insulation and it will too easily take damage.  You are also relying on being able to strip sheath 100% perfect with no damage to either black or white insulation. It is also not marked, so it is not legal to use.  And then you only have solid wire, which is more alligator wrestling.  

The other big problem with shucking NM is you are stuck with only 2 wire color choices. I stock 9 colors of THHN and 10 colors of tape. If you're buying just a few rolls of THHN, consider colors other than white black red. 




I don't know whether running exposed NM is legal in your shop. Conduit is very convenient for lighting. Do not hang anything on electrical cables or conduit.  If you want hangers for low voltage wiring or anything else, put them up there while you're up there. 
Your statements are inconsistent: one hand is hanging oversized conduit and running superfluous ground wires, the other hand is wringing over the cost of wire.  Make up your mind.  Either you're trying to save money or do a good job.   I wire my lighting grids with 1/2" EMT and 2 THHN wires. Easy peasy. 
As mentioned, individual wires inside NM will get damaged if pulled through conduit.
If THHN is much more expensive, you're sourcing at the wrong place. The big-box stores really getcha on small reels, and shamelessly overcharge on most other electrical parts too. (just not the ones you're likely to price-check). Big-box are not gods of pricing, or even selection; they are gods of being open at 9:30pm on a Saturday. They also have some freakish "Pied Piper" effect on consumers; it's almost impossible to convince folks to stop shopping at big-box.