Spellskite is about to gurgle upwards to $35, and then beyond.
I'm forecasting this moving quite a bit like Snapcaster Mage did. This could easily go over $50 medium-term if no new reprint comes forth in the next year or two. It's just so widely playable, and has demonstrated that it can reprinted without a collapse of value. Indeed, this shrugged off it's first reprinting like it was nothing. It was soaring upwards when it was reprinted in Modern Masters 2015. It lost a dollar or two, for a month or two, then continued it's tiered ascent into all-time high prices. It was perhaps the least negatively effected by the reprint of all rares in MM2015.
What's the ceiling on this card? I think $50, unless it sees no reprint for several years and it continues to dominate Modern decklists in perpetuity.
But if you're serious about hording some Spellskites long-term, your best bet is to go with Foils. Currently sitting (hardly moving, but upwards pressure on a perceived ceiling) at $35, Foil Spellskites (especially original New Phyrexia copies) will jump to $50-$60 within the next 6 months or so. Without over-reprinting, Foil Spellskites could be fetching $100 apiece in a few years' time.
This is such a good card: versatile, universal, useful, cheap to cast, hard to reprint. I think this will prove to be a widespread Modern and maybe even a Legacy staple, much like a Snapcaster Mage or Vendilion Clique. Get in now, and consider a long-term pick-up.
p.s. Happy New Year!