I wonder whether you were comparing like for like? I think there can be quite a variation in price between varieties, eg. looking at the Tesco site, "whites" at 52p/g and "finest reds" at £1.15/kg

On greengrocers, I used to like Hughes in Llandudno. Always a good selection of veg in top condition there. I don't know if they are still going but they changed to just wholesale supplying hotels, etc. in my time in the area.

Cheapest way we used to buy spuds was by the sack and there was even a farmer who played in a session we used to go to who was willing to supply - collect a sack from his car after the event. Of course we as household had more demand for spuds then.