Porter Cable via eBay offers the Porter Cable 15-Piece Impact Bit Set, model no. PCA2015, for $7.99 with free shipping. That's the lowest total price we could find for this set. It includes Phillips bits, flat bits, a 2" square bit, power bit, 3" magnetic bit tip holder, and more.
Update: Portable Cable via eBay also has the Porter Cable 25-Piece Impact Bit Set, model no. PCA2025, for $10.99 with free shipping. That's another price low.