When people think about building things with carbon nanotubes or with carbon nanotube enhanced materials they need to know that there is not much production of carbon nanotubes. There is less than 1000 tons/year of carbon nanotubes being produced and most of that is in a form that is like an unsorted powder. The material that has been woven into threads or sheets will soon be tens of tons per year and that macro scale material is not as strong at the macroscale. The cost of material increases as you require higher purity or a particular form of it.
Even material like Kevlar and regular carbon fiber is produced at the 10,000 ton to 100,000 ton per year level. Global raw steel is produced at about 1.2 billion tons per year and cement is at 2.6 billion tons per year.
There are many different kinds of steel and concrete. There is superstrong geopolymer concrete. Regular concrete can take 1500-5000 pounds per square inch but there is believed to be super-concrete that can withstand 50,000 pounds per square inch.
Fiber reinforced concrete using regular carbon fibers is advancing with long carbon fibers that are 500+ times better at resisting cracking and able to be 40% lighter.
There is a table of strength of materials and cost per kilogram.
Current thinking is that carbon nanotubes could get to 20 Gigapascals of tensile strength on a macroscale, but currently 9 Gigapascals is only for a few strands. They have to make a lot more.
Some believe and are working on nanokites to produce a lot of carbon nanotubes of unlimited length Longer carbon nanotubes allow more of the natural strength of carbon nanotubes to be available on a macro scale.
Nanocomp Technologies is making and selling spools of carbon nanotube thread and sheets of carbon nanotubes.
There has been strengthening of Kevlar by dipping it into carbon nanotubes to get to about 5.9 gigapascals. These improvements have been achieved at only 1 - 1.75 wt% of carbon nanotube content.
In terms of layman terms you can think of the strength as multiples of the strength of Kevlar or Zylon.
Currently even powder like carbon nanotubes can cost $45-150/kg, but it goes up from there for purer forms of carbon nanotubes.
Some in Italy and other places are considering building with carbon nanotube enhanced material to make longer bridges or to reinforce old bridges.
CNAno Technologies 500 ton/year factory has received ISO 9001 certification.
Carbon nanotube nanostitching can reinforce sheets of material and reinforce existing materials at their weakest point for airplane skins and other applications.