A Modern simic darling, Voidslime is arguably the best of the 3-mana counterspells, with the possible exception of Render Silent (which is a good spec, too, but with far larger supply).
From a financial perspective, Voidslime sits in a sweetspot in Magic history: Modern legal but very old. It has also, along with all other counterspells, benefitted from the demise of the UU Counterspell in favor of the 1UU Cancel at about the same time (this is a major distinction between the Modern and Legacy formats). This shift to weaken blue opened up the 3-mana counterspell space, and Voidslime has been the blue-chip in that space since that time.
Go for a playset of original (Dissension) Foils. They're $17 apiece right now, in a few months we're looking at $30-$40 apiece. Non-foils under $8 are a great pick-up, but for short-term speculation only as Conspiracy 2, Eternal Masters, Modern Masters 2017, or even the coming Standard-legal blocks could hold a reprint of Voidslime.
Voidslime's flavor and flavor text is extraordinary.
For all the non-foil movement in recent years, the foils have only hardened up and started up the foothills...