It is ever thus.
Developer has a cost per build plus a profit margin that has a top and bottom line. Bottom line is cost plus minimal profit - not good for banks and shareholders. Top line is best of all.
After all the banking costs etc are agreed the only “wriggle room” to get the top profit is to get it built for less - take it off the trades / suppliers. So, they tell you they’ll pay x per metre- if you do it they win, if you do it shite they win coz they don’t pay, sometimes even contra charge you for someone else to put it right.
Negotiate the rate before you begin, remember, we can all stay at home to earn f**k all. No hassle.