Set Number 5487 Bricks & More parts box, containing 700 bricks, is reduced to just £7.13 at Asda at the moment. With an RRP of £21.99, that's a whopping 67% discount! You'll probably see that the shelf price is £14.25, but it rings up at £7.13 price at the till.
I believe they also have a few other old sets for a similar discount (i've heard rumours of the 3221 Lego City truck for £7-ish!), but stock may vary depending on your local Asda store.