Cost for 2020 Games gymnastics venue more than double now

Big surprise!

They added the initial price reflected only the construction cost, while Shimzu’s bid included design cost as well.

Huh!!?? Cut out the design then. We probably won't notice.

but when a foreign designed stadium doubles in price because of japanese construction companies greed there is a public outcry?

Cost is not "doubled", more like "Real" cost has finally surfaced. Happens all the time in Japanese companies, super-experienced-know-it-all-buchos, presenting best-case scenario project just for approval, because whatever happens later, they can ganbaru the hell, then cost "doubled".

The 'kakunin mitsumorisho' or 'confirmation estimate after the project is delivered' is one of the cornerstones of Japanese business, and is subject to intense negotiation. It is very different to the philosophy of providing a realistic estimate and sticking to the terms of it.

Bid and propose cheaply, win the project, then get your money back afterwards.