I believe this is the lowest I have ever seen Mass Effect 2. Steam has recently had this for $9.99. Origin had this two days ago for $6. Currently D2D has it for 75% off or $4.99. You can combine this with a referral code for an additional 15% off.
It seems that there's a 50/50 chance that it'll dip this low for the Steam Summer sale, however EA did pull Crysis 2 - so who knows if they'll offer sales this time around on Steam.
Additionally, I believe this can be activated on Origin, not Steam.