If you look very carefully you might find some great offers out there. I go this pair for £12 at Blacks. (Buy one, bet the other half price). This is pretty good considering they are around £10 - £12 normally.
Be quick if you want one as there weren't that many when I bought these and there wasn't a huge selection in terms of colour. I bought a stainless steel and white version. Both colours were fine for me.
My orange mug had already survived a few knocks and even coped falling out of my S - Bag while on my commute and rolling down the road. They are very tough, hold 300ml and keep a drink hot or cold for ages.