Already put the contractor contact in my first post, please take note the itemize cost that I got at that time already no longer can be used as a reference ya.
Good luck.

2 bedroom plaster ceiling for 3k ? What is your bedroom size ? What type of plaster ceiling

This kind of out of standard and the price plaster ceiling is like 3-4 times more than normal

This is bad. This is one way contractor slaughter unexpected people

Just saw your msg, that time when imam doing my house renovation, the price for wiring didn't include the socket, I have to buy socket plug, switch. I am OK as already align before start.
I will write down and communicate with him again before they start, What is included and what not included.

Extension area that you means is it extended with slab or roof which is similar as my kitchen?
That time when I extended my kitchen, with slab, that price included plaster ceiling (flat type No cornice). As my kitchen only 5 feet, 18 feet x 5 feet means only 90 square feet.
That time when you survey for contractor, is the price imam quote you cheaper then others? Or nearly the same?reason I asking as From there we will know roughly where to price go. Try to gather some data/info first before negotiate with him.

If you ask me, In my opinion, that is his fault as he miss out in his calculation. That time you give him to do as you agree with this price. Try to negotiate with him if can get win win?