User blog comment:X-Machinist-X/Dishonored random stuff/@comment-192.0.189.206-20151205060413/@comment-1500935-20151206201559

Be careful of which online retailer you get your key from though, as some simply scan keys from retail editions and resell them, or sell keys regardless of region restrictions (illegal in both cases), and Steam can at any time revoke those. You can also wait two weeks for Steam's Winter sales and be sure to get the DLC at low prices.

Also as X-Machinist-X said, the GOTY will probably be cheaper than the sum of all DLC.