I highly suspect there's some creative accounting in this project. Why? Because even the whole of the North-East MRT line had only cost S$4.6b and with so much of it being also underground.
Construction cost is a small part. It is the 'land cost' that contributes to the $4.3b I guess.... Just like building loss-making HDB flats on 'valuable land'.
I thought the MCE is entirely underground as well? So, that's why I'd say Creative Accounting because most of the "land cost" would have gone back to SLA, the "left hand to right hand" approach.