I'm not sure there's much variation in the price of sugar. It's one of those 'known cost items'/loss leaders and often the cheapest place for a standard kilo of granulated is a supermarket and it's the same price everywhere.
If you want soft brown, I think Aldi and Lidl is often cheaper than Sainsburys etc, especially if you can get a kilo bag.
If you want caster sugar and are just using home baking quantities (it's probably too faffy to do more), you can turn granulated, which is much cheaper than caster, into caster sugar if you have a food processor. But you have to watch it carefully, because if you go too far, you get icing sugar instead.