That's a great question! When you're looking for the most affordable option for a basic outdoor trash bin, plastic is almost always the winner on upfront cost. You can find simple, no-frills plastic bins at major big-box stores, hardware stores, or even discount retailers for often under $20. These are typically made from durable, weather-resistant resin and come in standard sizes like 20 or 32 gallons.
Another very budget-friendly route is a basic galvanized steel or aluminum can. They are incredibly simple—just a metal cylinder with a handle—and can be very inexpensive, though they may rust over time without a liner. For the absolute lowest price, check store brands or generic versions rather than name brands. Also, consider a bin without a lid if you're in a low-wind area and don't have pest issues, as this can shave off a few dollars.
The key is to balance affordability with a bit of durability. A very cheap bin that cracks in one season isn't a good deal. Look for UV-protected plastic and sturdy construction even in the lower price range. Your most affordable basic option will be a simple, medium-sized plastic bin from a large retailer.