What is a economic way to store/buy ingredient for salad
I myself love to eat salads, but can't ever seem to store them for more than a week. For me, I  only buy salad when I want to eat it. However, there are some tricks for getting more miles out of your garden treats. Here are two: 

Remove as much air as possible from your storage container
This is kind of a no-brainier, but I have had people in my household set an open plate of salad in the refrigerator, and wonder why it spoiled over night. Air and moisture are the enemy and make sure to protect your goods. You have probably seen on commercials special bags or containers that claim to remove air and keep vegetables fresh. Be wary of these products, I have tried some and usually don't get the features promised. 

Don't cut salad with a metal knife
Perhaps a lesser know fact about lettuce is that cutting it with certain metals accelerates the oxidation process. This simply means that your lettuce will turn brown faster if you cut it. Try using a ceramic knife or rip your greens when preparing to slow down on browning. 

To answer your questions about storing. I would recommend not keeping your greens for more that a week, and store them in within the designated bin marked in your refrigerator. This bin is designed to remove moisture and keep your veggies fresher longer.