Toys'R'Us has a coupon for $5 off $25 Lego purchase that's valid until 3/12/15.
I tend to stay away from announcing offers from the Lego company because they're different by region. But this month's offers -- at least in North America -- sound fairly interesting and open to being "gamed" a little.
North America March 2015 LEGO Offers:
-FREE LEGO Bionicle Hero Pack with any purchase (valid March 1-20)
-FREE Elves Azaris's Magic Fire polybag with any LEGO Elves purchase (valid March 1-31)
-Double VIP Points (March 14-19)
If you get Aira's Creative Workshop for $9.99, you end up with 134 pieces (9 from the Bionicle set and 27 from the polybag) which is $0.08 per piece, and if you buy this after the 14th you get double points for that purchase. Not bad for $10.