One problem with the math for the ROI calculation is that catching a bug with a test is counted the same as catching a bug while you're typing the buggy code. Type checking lets you catch bugs sooner, thus more cheaply.

Another problem is that it focuses on bugs that make it out to production. Maybe for some projects production bugs are so costly that pre-production bugs are relatively negligible, but for most projects you can't treat pre-production bugs as 0 cost.