well I came across your questions in google searching for pure thc extraction and came upon an idea that could hold some merit.
for your first questing I'll answer with: I don't really know but, what I would try is: using a mild air pump I'd draw the fumes of a commercial vaporizer through isopropyl alcohol, the best way ,I think, would be to arrange the alcohol in a long upward sweeping spiral tube. then drain the thc laced alcohol into a pan to evaporate then scrape up what is left. if what is left is not pure clear crystal thc then more complicated extraction would be needed. by the end of something like that you would know the difference between acids and alckaloids and be able to extract dmt from common grass(which might prove more profitable than pure thc)
for your second question: you would have to use so much weed to extract so much pure stuff that you would be hard pressed to find anyone willing to pay you enough to bring you a profit. rather than wasting your time with all this extraction bullshit, why not just grow damb dank weed and sell it by weight? but there is this new e-cig invention that people might be down like a sad clown in china town to load up with some pure no-shit weed extract combined with some glycerin to smoke up from a tiny pocket vaporizer.
as for your third question: it would cost a kings ransom. might be on the level of a pure uncut kilo of peruvian cocaine