They're just cans... a normal trash bag weighs about 5lbs. Save as many as you can stand having around the house, whether that's a bag or a trailer load. When you start thinking to yourself "these cans are everywhere!" that's when it's time to take them in. Prices from yard to yard probably wont vary enough to matter where you take them unless you have the ability to bail them because loose they are too light to really achieve any scale at shipping them.