Newegg.com has a couple of promo codes for the XCOM games on PC / Steam:
XCOM: Enemy Unknown - $4.99 after promo code EMCWHPA24
XCOM: Enemy Within - $5.99 after promo code EMCWHPA23
These are physical versions but come with a Steam redeemable product key. I believe the two game combo has been cheaper but this is the lowest I've seen the Enemy Within upgrade. Free 4-7 day shipping.
Credit to ZedAvatar on Reddit for posting these.