I built one 2 years ago, was 4m x 5m. Got all the stuff from my local timber merchant, and it was around the 750 pound mark iirc. Don't underestimate how much it'll cost for stuff like screws as well, if your doing it properly, ie joists every 18", it'll be about £50 for screws, and as they'll be 3-4", you'll want a proper impact driver to do them. That'll be another £100.
Takes a wee bit of looking after every year as well, and I have to say, even though I had a big enough garden in the last house, a deck of that size looked huge. I fitted balustrades most of the way round it as well, I wouldn't do that again, but my daughter was just walking at the time, and I didn't fancy her falling off the edge.
Just some things to consider.