I do not know what HF did or didn't do or what they had planned to do.
I can say that test chips are typically used to prove out sections of a new chip/new technology. It is a small fraction ($100K range vs $Ms) of what a full tape-out costs and is used typically to mitigate risk. It isn't typically viewed as an overrun, but more as money spent to reduce risk.
The cost overruns in 16 NM come from the test wafers?
From a business or bank side when a developer asks for x dollars and you pass it then ask for more cash was my thought process.
I was reading Bitfury scraped 16 NM because they ran into problems.
Its about 80% more to develop 16NM compared to 28 but doesn't include what a developer can import from a previous chip?? It depends on how far along HF got.