Lettuce is the one and only vegetable that can't be preserved by any method. You can't freeze it. You can't can it. You can't dehydrate it (well, you can but you can't use it for anything afterwards.)
When frugality boards talk about 'the price of lettuce', we're talking about a very real commodity factor. Since lettuce can't be preserved, the price in the stores IS the going price for that commodity. It is a very real example of how the marketplace (not just the grocery store) works.
Lettuce will last a bit longer if you preserve it properly. Tupperware has an excellent container that will help keep your lettuce fresher longer and it DOES work. Don't use a knife to cut your lettuce, that will help it turn brown faster, always tear it. Sometimes storing it with a wet paper towel will help it stay fresher longer.
Lettuce will keep longer if it is kept dry. Don't wash it until you are ready to eat it. Add a paper towel to the container to absorb the condensation. (This applies to spinach too)
Also if you buy a head of lettuce, if you keep the core in it and peel leaves as you need them, it will last a lot longer than if you take the core out right away.