What’s the cheapest thing in a jar one could buy right now?

I’m looking to get one or two jam jar style jars and I thought about getting some cheap own brand jam or pickled onions and consuming them/decanting them into some plastic containers I have, and then giving the jars a good wash.

I was wondering what you would consider to be the best return on investment in terms of volume of glass jar is at the moment. I think you should consider that things like marmalade will be messier to empty than say beetroot, even if it is cheaper.

If you want to get super specific, my nearest shops are Morrisons and Lidl