Anything sold for food use has no agricultural residues so no chemicals. It's the law. Food Safety Act 1990 and all that, supporting regulations available on request. Compliance is very high, there are of course some pesticides popping up in food here and there, but it's rare. General compliance is good enough that you can ignore it and grow stuff from food crops if you want. Source? It's my job, and all that.Yes, non- organic seeds are cheaper. They may well have been prepared with the use of chemicals to prevent rot or sprouting. Not a good idea to consume sprouts or greens grown from these seeds. It is better to buy organic sprouting seed...not much difference in the cost. A small packet goes a long way and germinates for years if kept dry and in the dark.
I have saved 'heirloom' (open pollinated) seeds and stored them in brown paper bags with success.