If your jobs sometimes go sideways once they are underway, there is a specific step that tends to be missing, and it is the one contractors cut most often. Preliminary engineering is a defined scoping phase, done before the main work is priced and committed, where an engineer confirms what the actual problem is and what solving it correctly will take. Its whole purpose is to make the real scope visible before anyone spends money against a guess.
What you are actually choosing between
Bid from the client's description
What it is. You price the job on what the client says they need and start work.
The trade-off. You save a line item and a few days at the front of the schedule. Those savings are real and they are small. Almost every expensive surprise on a job, expanded scope, wrong equipment, schedule slips, and commissioning problems, traces back to nobody paying to define the problem before designing the solution.
Scope it informally during the bid
What it is. You walk the site, ask better questions, and adjust the number based on what you see.
The trade-off. It catches the visible problems and it is free. It does not catch the ones that require analysis, which are the ones that cost real money: coordination requirements, equipment compatibility, and lead times that only surface once someone models the system.
Run a preliminary engineering phase
What it is. A priced, defined phase where an engineer confirms the true scope, the right equipment, the lead times, and the order of operations before the main number is final.
The trade-off. It is a real cost and a real delay at the front, typically a small fraction of the project. In exchange, the full picture is on the table before the bid is locked, so you bid accurately, order the right gear the first time, and walk into commissioning without a scramble.
The cost you are moving, not avoiding
You do not avoid the cost of preliminary engineering by skipping it. You move it to the point in the project where corrections cost the most. The same information shows up either way. The only variable is whether it arrives while you can still plan around it or after the number is locked and the wrong equipment is already on a truck.
There is a relationship payoff worth naming too. When your engineering approach includes real scoping, your clients stop getting hit with change orders on your projects. The number you gave them holds. Over time, a reputation for clean, predictable work is worth considerably more than the savings from cutting the step.
So when you compare engineering proposals, the question is not which one is cheapest at the top. It is which one defines the scope before it commits you to a price. A low bid that skips preliminary engineering is not lower. It is the same cost, rearranged.