The Calphalon is anodized aluminum in one form or another, while the A-C is tri-ply stainless. I would suspect that the manufacturing cost to make a tri-ply pan is higher than the cost to make an anodized pan.
Also, the A-C SS is dishwasher safe and has a much more durable surface inside and out than the Calphalon. The anodized finish of the Calphalon can be scratched or worn off, exposing the raw aluminum.
They both cook very well and some folks have a preference for one pan material vs the other.
Finally, All-Clad charges more because they can. You can buy comparable quality tri-ply stainless cookware for a lot less. Some of it from Calphalon.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan