codeeng,
As previously stated, there are many major tank builders that can do this type of work. I would stick with large companies CBI, Matrix, ATV, PTC, and the like. If you are thinking that 70-80% additional is a good guide, I would disagree. There are a number of special difficulties with steel containment that need to be addressed.
1. Will the inner tank share the bottom with the outer tank?
2. Will fitting penetrate both shell walls? If so, how will differential expansion be handled?
3. How will the outer tank be roofed to eliminate rain accumulation or will it be left open with it's own draw-offs installed?
There are many questions like this to be answered during design and quote. Since the outer tank is larger than the inner tank, depending on the answers to these type of questions, you could have an additional cost of up to 150%.
Best of Luck