Formal methods encompass a very wide range of techniques that vary wildly in cost. When the appropriate technique is applied to the appropriate problem, then it

Formal methods encompass a very wide range of techniques that vary wildly in cost. When the appropriate technique is applied to the appropriate problem, then it can certainly reduce costs, even significantly. But like I wrote in another comment, most of the cost of building software is determined by what we build, not how. Tools and techniques, even those that are effective, can only help so much.