X-Com: Enemy Unknown on Amazon for $9.89
Enemy Within expansion also 9.89 on Amazon
Great prices, I picked up Enemy Within through this. If you get it through Amazon you can go to your "games library" page to get the key without having to go through the nonsense of installing amazon software to do so.
Some deep discounts on at least a few linux-compatible titles:
X-Com Enemy Unknown Complete $25.00
Europa Universalis IV Digital Extreme Edition $11.24
Pajama Sam - No Need to Hide When its Dark Outside $1.74
Cities in Motion Collection $12.49
Freddi Fish and the Case of the Missing Kelp Seeds $1.74
Knights of Pen and Paper +1 Edition $3.74
It's been a while since I've purchased a game from Amazon but I know at least some (most?) of the titles come with Steam keys (DRM: Steam under the title). I don't know if steam keys can be obtained for the drm-free versions--please use due diligence and research before buying.
EDIT: Newegg has Enemy Unknown for $7.50
Enemy Unknown is on sale for the same price as EW at Amazon ($9.89). The Slingshot and Elite Soldier Pack DLCs are still full-priced ($7 and $5, respectively).
The Complete Edition is available for $33.48, but that is slightly more expensive than buying all pieces separately.
$9.89 (EU) + $9.89 (EW) = $19.78
$6.99 (Slingshot) + $4.99 (Elite Soldier) = $11.98
$19.78 + $11.98 = $31.76
For comparison, here are the prices from a sale 6 days ago.