Good people and good companies that invest in their employees and equipoment don't come cheap, but they're far more efficient than the low-wage fly-by-night low-bid jokers that you'll get if this law passes.
What we (the public) will be paying for is Little Joey and Uncle Buck, with two shovels and a broken down backhoe and their sub-standard work, huge cost overruns, illegal hiring practices, jobs not done on time, jobs not done according to specs, dangerous working conditions, who leave the state and change their names the day before the inspectors show up. We end up paying three times what it would have cost if we had hired reputable companies paying prevailing wages to employees with the training and experience to get the job done within budget and on time. Besides, we shouldn't pride ourselves paying paupers' wages to people doing the work of the People, like building bridges, schools, hospitals, etc. because we'll only get what we pay for.