In some cases you bid the overtime into the job, but in most cases you don’t. Overtime is like a two-headed snake. It cost you time and a half, and you loose productivity. If you work a man 60 hours in one week, you loose 50% of his productivity for the 20 hours overtime.
1 man 60 hours in one week = 50 hours of productive work.
1 ½ men 40 hours in one week (the same pay) = 60 hours of productive work.
It is much more cost effective to add more manpower than to work overtime.