Trinisphere has an eternal demand, and is so wonky that it will likely not see another reprint outside of supplementals or Commander, as it did in it's FTV: Relics reprinting. A Commander reprinting would hit the non-foil value quite hard. This is a very powerful card, it reminds me in finance terms of Oblivion Stone. I also like that it's legal in Modern, which is the eternal format with the most room to grow, in my opinion.
If you can find these non-foils under $5, they're a good short-term hold. Why short term? These seem ripe for a Commander reprint. If Wizards jumps onto the Tiny Leaders bandwagon in an official capacity, you can be sure this will be in a Tiny Leader product. For these reasons, I'm for picking up Darksteel Foils at or below $20 (as a $40 price tag seems to be solidifying quickly) and FTV relics at or below $7. The FTVs will behave much more like the non-foils from Darksteel, UNLESS we see a non-foil supplemental reprint (like a Commander reprint), in which case the FTV foils will differentiate themselves from the Darksteels non-foils, which will fall hard with the new copies. A non-foil reprint will help the FTVs slightly, and will hardly touch the original foils, in fact it might increase the cost as newer players get exposed to it and some will decide they need a foil. The foils are a long-term hold, but don't be surprised if the non-foils breach $10 per card in a few months. If they don't see a reprint in Commander 2015 (November, I believe), then they're golden for a year, in which case they will likely cruise to $12 by next fall for original non-foils.