QSS build path: Why is Mercurial Scimitar So Expensive?

As I was updating my Shyvanna items to include QSS, when I noticed that it builds into only 2 items. Mercurial Scimitar at 3700 gold, and Dervish Blade, an AS item that costs 2600 gold. That is a huge difference in cost, most of which is in the final cost. Stinger and BF Sword aren't that much different in cost. So why is the scimitar so much more?

Is AS really worth so little compared to AD? Maybe the cost difference should be normalized.