I've read those safety standards and I disagree. They don't prescribe much that isn't standard practice. The only thing they require that isn't commonly done is very extensive testing or formal verification. They high expensive mostly comes from proving compliance to external auditors. You could have 90% of the results with 10% of the work by just testing your software very thoroughly but not producing all the documentation required to prove compliance to the standard. SQLite for example is at least as rigorously developed as SIL 4 software.