A reasonable explanation for this price jump is the most popular commander from Battle for Zendikar, Omnath, Locus of Rage. Since being released in October, copies of Boundless Realms have been flying off of the market. With the Locus of Rage’s Landfall ability to put a 5/5 Elemental creature token into play for each land that comes down for you, it's not surprising to see Boundless Realms become a staple in that deck.
The somewhat surprising thing is that Boundless Realms foils have remained around $4-4.50. They hit almost $5 at one point but it's the non foil that's being bought out. Boundless Realms is easily a card that could see a reprint in a Commander deck, so the foil feels like a safer long term investment.
Risk: Low. Organic demand alone will make sure that Boundless Realms keeps a price over $2 unless a reprint happens,especially in a full fledged expansion set. Foil risk is much lower, except if Boundless Realms does get reprinted in a major release.
Price target: $5 seems like a decent ceiling for this card if supply dwindles. We've also seen commander related buyouts happen before. $3 to $4 seems like a better long term price, but additional Landfall cards being printed that prove good in Commander could put a lot of yet unforeseen pressure on this card.
Looking to double your money on foils seems reasonable in the long term. $8 seems like a fine target price. This is a card that sees play in far more than one deck so there will always be some demand for it.
Term: Long. Cards that see consistent play in Commander are always great investments barring a reprint. This would be a very easy card to reprint, but if it isn't, then you could make a little money out of this. If any more green Landfall centered commanders come around, this will be a must have.