Ive always found bosch the best compromise, they are cheaper than PIA blades (which ive heard very good things about) yet still perform very well.
Some places seem to charge silly money for bosch blades, if we (a small motorfactors) can charge £9.99 a pair im sure halfords could knock that extra £10 profit off, they probably buy them cheaper than we can anyway.
Asda now stocks quite a lot of car care products and generally destroy us on price, by the looks of the products they keep they seem to have went to our main supplier and basically said, give us the best selling range. They are generally selling products for a few pennys more than our cost price.