I don't think I've ever seen it cheaper than $20 new in physical form. If you don't care about having the CDs or strategy guide, you can buy both the digital version of the games for $14.99 from the Blizzard Store.
Looks like it's been $10 at Amazon a few times. If you've waited this long, might as well put it in your cart and save for later and monitor the price for a few weeks.