If you want a three-seater Robin Day-designed Habitat Days Forum sofa, you'll be looking at a price in excess of £2,000 at the Habitat website. But if you shop at a certain auction site, you can get one for much less.
In fact, eBay is always a good place to look for this 1964 design in either sofa or armchair form, with many examples available at discounted prices and in decent condition. With that in mind, it's always worth checking first before you buy new.
The sofa above is in dark brown leather, with ash surround and in 'good condition', sold with original leaflet and habitat leather cleaning and 'feeding' kit. It's up at £950 or you can make an offer.
Alternatively, opt for an armchair, this one is in cream leather with ash surround and is said to be in 'mint' condition and just a year old. £650 is the price, but again, you can make an offer. See the site for more examples.