p.s: our doctor told us if we cant get the info about the tank cost we can get it from the internet and then convert the currency so the material cost does not matter from which country
The info you are asking for is very unique. These types of structures are built one by one, not in a production line. The costs vary all the time simply because each one is different. It's unreasonable to simply grab a cost off a random project and assume that's the cost simply because it's on the internet.
If the tank is constructed it will either have to be constructed in the home country and transported to the site or in a foreign country and transported to the home country and than to the site.
Costs on the internet are from very specific scenarios that may have nothing to do with your project.
In the first place, not every country has the ability to build such tanks. To do so would in many cases require investment into a one-off tank which would be much more than an internet quote as the equipment cost would need to be included in the price.
In the second place, those countries which can build it and then transport it would not only face enormous transport costs, but would likely encounter tariffs that could double the cost. That's unpredictable since not every country has a most favored nation clause with every other country. In some cases, these materials won't even be allowed to be exported or imported.
If your doctor means, he or she doesn't care what it really costs, so that's why "the material cost does not matter from which country", then why not make up a number and move on?