Aluren is on the Reserve list, it's waiting in the wings to become an established Legacy archetype (it's fringe right now), it is combo city, Wizards keeps printing playmates, and it's cheap enough right now to encourage brewing. A deck focused on Aluren will take more than one Aluren, likely 3 or 4. So if this deck breaks out, it'll mean a price spike for sure; demand can turn to full blast in no time, and there is just about no slack in the supply of this card.
Most old-timers know about the power of this card, but to be fair: it's clunky. It takes some doing to get a competitive combo deck out of this card, but the temptation to do so is blindingly obvious. All small creatures are free and instants? Shenanigans!
Aluren fever struck Magic almost 3 years ago (early 2013). For a few weeks, Aluren was worth $15-$20. I think it's headed into a second spike right now.
Old cards typically won't spike a second time with a post-spike equilibrium price less than the first... especially not for Reserved-list cards. There is only one way to go for Aluren, as long as a competing card doesn't get printed: UP and then UP some more.
I think this will start with a smaller spike up to around $10, and will peg that price pretty well. Within a year, I suspect it will start a second swing up past $20. I don't think the next Aluren spike will fall as quickly or as completely as the first spike did, as the slack in supply was taken out during the last price spike.
Grab these around $5.00 as a long-term hold.
Momir Vig, Simic Visionary, Glimpse of Nature and Gaea's Cradle come to mind as excellent cohorts for this powerful enchantment. The "instant"-ification of creatures also has some interesting combos.