The best deal for Organic Spinach and Organic Spring Mix is at Costco. Both are $3.99. If you don't have a Costco membership, I feel the second best would be at Save On their brand Organic.
The key to making your lettuce and spinach last is proper storage. Turn out the packs into glass containers lined with paper towel and then top with another sheet of paper towel to absorb excess water. This will give you a few extra days of freshness. If you leave your greens in the original container, they will rot faster. Spinach is great in your smoothies and Spring mix is a great base to any salad. Eat them daily and eat them often.