Potatoes will keep (raw) for months in a cool dark place. I would never throw them away. Though I do wish I hadn't bought a second bag of old spuds then I could justify digging the new.
Other things that are cheaper to buy 'two for..', well the cheese keeps in the fridge and I eat loads, 2x4pints of milk has to be decanted and frozen.
There has been a campaign against food waste and thinking about it I can't remember seeing any of those stupid 2 melons for... etc in Tesco recently. That used to annoy me - some fruit is great when ripe, useless before or after.