Walking Ballista had a great first week in Standard, and as such has more than doubled in price. In many ways, the card is very reminiscent of Hangarback Walker. They're both artifacts cost at XX, they both use +1/+1 counters for some effect, and they were both initially underpriced.
People played it safe pre-ordering Walking Ballista at $5-6, though its week one performance has turned it into a $12-$13 card. Not only is the card powerful in Standard, but it's also being discussed with regard to Modern implications. I played against multiple Tron players casting the card in the SCG Classic this weekend, and I've heard rumblings about utilizing it in a new version of Affinity. Modern play is generally going to impact foil prices much more than non-foils though.
All that said, I expect Walking Ballista is at or near its price ceiling. Aether Revolt will be more opened than Magic Origins as is a set with Masterpieces. Should the Ballista see play in multiple Standard decks it could theoretically hold steady around $10, though over time I couldn't see it holding at $15-20. I'd be happy owning a set, but would be looking to sell off extra copies in the next couple weeks.