Well it's not difficult to see that you need around 12 square metres of carpet. The problem is with trying to give a guide price for 1 square metre of carpet. I've just taken a quick look at the Allied Carpets website. Their prices range from �1.99 to �84.99 per square metre. So that makes 12 sq,m. cost anywhere between �24 and �1020. That's quite a range!
In practice, though, if you hunt around some of the smaller shops (or market stalls) that specialise in selling roll ends, you shouldn't need to pay much more than �100 for a perfectly acceptable carpet. (You might well get something for quite a bit less).
Remember that the cheaper carpets are generally only intended for light wear (e.g. in a bedroom). If you want something that will last a long while in a room where dozens of people will be constantly walking across it in their outdoor shoes, you might need to pay a bit more.
If you're really on a tight budget, check out the local auction rooms where they have house clearance sales. You might pick up something perfectly acceptable for �10 to �20.
Chris