LoE from PoC to production

I am not sure if estimation is an art or science but its tough. Usually the level of effort required to build a production quality application is approximately three to four times (may be more) the effort required for the proof of concept or prototypes. Why?

1. You would need to translate the If-Then-Else in PoC to a proper pattern in prod.
2. You would need to translate the "invoke this" to an independently runnable task.
3. JUnit classes are missing in PoC.
4. for with in for is okay for a small dataset but this won't be true in prod.
5. PoC doesn't have to deal with multiple and heterogeneous systems.
6. That hidden hard coded value won't hide any more.
7. Did you add code reviews and documentation?