Pre-packaged produce is pretty common at most grocery stores. And while it might seem convenient to reach for a sack of onions instead of just grabbing a couple off the shelf, odds are it's going to cost you much more. Most stores price their pre-packaged produce and other foods much higher than items sold individually. The worst part is that it really doesn't even take that long to grab a couple of items off the shelf and go weigh them yourself.
However, that's the price you pay for convenience, so the next time you're out doing your shopping, just be aware that pre-packaged foods often cost much more than their counterparts sold individually.