Gemstone King via eBay offers this Sterling Silver Mother of Pearl Mother Daughter Heart Locket Pendant 2-Piece Set for $29.99 with free shipping. That's tied with last month's mention as the lowest total price we've seen. (It's a current low by $49.) It features a 3/4" mother locket with 18" chain and a 1/2" daughter locket with 16" chain.
If you want it guaranteed in time for Mother's Day, you can opt for 1-Day Shipping; however, it adds $37.