First, and this is a bit late to the party, Hardened Scales is exploding in price. Wizards has put a lot of +1/+1 tech into recent sets and both Standard to Modern playable decks are brewing. This stacks well. It's rare. It's splashable. It oozes casual apeal. It's reminiscent of that perennial favorite: Doubling Season, but costs a fifth to cast.
Don't bother above $4.00 apiece right now, unless you need it for Standard play right now or if we see a roll over of the price graph and the emergence of a second upgoing feature.
If this archetype (+1/+1 counters) becomes a force to be reckoned with in any Eternal format, then my candidate for best sleeper in the last few sets has got to be Avatar of the Resolute. Avatar of the Resolute reminds me very much of Tarmogoyf. He stacks very well. He is cheap to cast, versatile and scalable. Reach is a big deal for mono green aggro. He is just the bees knees.
I wouldn't be all that surprised if he saw $6-8 while in Standard if the pieces of the budding +1/+1 archetype keep falling from heaven (like Hardened Scales and Managorger Hydra) . I want to play him all the way back to Vintage. NOW is the time to buy in on Avatar of the Resolute. Anything under $1.50 a piece is a fair price, but don't expect that price to last beyond the next few weeks.