I don't use Undermine (but I've started looking at it lately), but the way I figure it, I will buy elementium ore if it's 35 gold per stack or less, maybe go a little higher. If you have the worst luck in the world, you'll only get 4 low quality gems. If all 4 of those are alicite, then if you auction them uncut, you'll probably make 15-18 gold at the most, so that's kind of a gamble. However, that's the worst case scenario. Generally you can expect to get at least 5-6 low quality and 1-2 rares from a stack of 20. Plus my guild has the Bountiful Bags perk, so I get a lot more.

I do the carnelians->spikes->greater celestial essences, and again with Bountiful Bags, I get more essences, which sell for 65-90g each. Another big money maker is inferno rubies. I never sell uncut, I always cut them into bold, brilliant, or delicate, and if I list later in the afternoon or evening, I can get 135+ per cut gem. That one gem is worth 3 stacks of elementium ore.

There are about 7-8 other popular rare cuts I do that are fairly reliable, but nowhere near as lucrative. Still, any cut gem that sells for 35+ has paid for the ore I prospected it from. Really, I make a profit as long as I sell every rare gem (cut or uncut) for more than 15g. Low qualities I sell according to the market: make jewelry and DE, or sell for the JC daily. Low quality and hypnotic dust have both kinda bottomed out on my server though, so I'm sitting uncomfortably on a stockpile I'm not sure what to do with.