Good panniers / racks will last longer and be more dependable than cheaper ones. If you're relying on your bike in place of a car it's worth paying a little bit more for that reliability.
Let me paint a typical commuting scenario for you:
You're five miles from home zipping down a hill in the pouring rain and have one of your panniers come loose and swing into your rear wheel. Your dress clothes are now soaking wet (likely ruined), you've lost a couple spokes, your lunch is crushed, and you're late for work. Probably 60$ to replace the clothes, 10$ to buy lunch, 3$ to ride a bus the rest of the way, maybe you've ruined your rear rim, at least you'll need a couple spokes and a lot of time truing, plus you need a new set of panniers. Plus you have to figure out some way to make it through the rest of the day with no dress clothes in your office. That 50 or 60$ savings doesn't look so good now, does it?
I think that for buying groceries only, it makes much more sense to go with a used bike trailer. You can pull more stuff behind you than you'll ever get in panniers, and the weight is off the frame of the bike.