Material costs can be kept down by innovative design and a little bit of lateral thinking.
The olympic stadium was built under budget and within time scale by utilising material that was largely already available from stock and could be reserved for 12 months while the design was finalised.
The same design team and structural engineers used the same source of supply that they used for the emirates.
Little known fact - both stadium have common sizes within their structures and, coincidently, the emirates incorporated the same size as the Cannon Street building pictured earlier (it's above the underground station and won an award, but I can't remember it's name).
Wimbledon no 2 courts roof will include some of the same sized tubes as the olympic stadium also.
Wembley was designed around material that was all specifically made for the project so they had to wait until it was made before they could start fabrication - and agreed to pay the price ruling at the date of dispatch instead of agreeing a fixed price.
And the Etihad (commonwealth stadium as it was when built) uses a type of tube that could only be sourced from one manufacturer.
John Lennon airport used elliptical tubes (oval instead of round) which cost about twice that of circular sections.
But the aesthetics were considered to be worth the cost on both schemes (how many people noticed the oval steelwork at the airport before now ?).
Spurs look to be using a combination of precast and slip formed concrete in the terracing (which I guess must have been the cheapest option,
but their roof includes some very unusual tubes that will be costing a lot simply because there are limited number of suppliers.
Forgemasters in Sheffield could produce what they want - but my guess is it will ultimately come to the UK to be fabricated near Bolton from either Italy, Germany or the far east.
I don't think they have finalised the roof design just yet so the material won't have been ordered, meanwhile steel prices have gone up 10% worldwide plus another 10% due to sterlings current weakness.