Hi, I just finished a decking 8.5 x 3.8 m. Materials cost just under £ 1000 from local timber yard (4x2 treated C16 joists and 32 mm redwood boards). I did it with my son-in-law in 10 mandays. It was our first decking and was built around a spa, so quite fiddley in places, so a professional should be able to do it faster than that.
Hope this helps, Andy
Hope this helps, Andy