Suppliers who all describe themselves as prototyping companies do genuinely different things, and the gap between the cheapest and the most complete is an order of magnitude. Deciding which kind you need is a question about how much engineering you are keeping in house, not a question about parts, and getting it wrong wastes either money or months.
Four kinds of supplier
A parts shop makes what your files say. A prototype builder makes the parts and assembles them, catching clashes as they go. A hardware prototyping company adds electronics, firmware and integration. A development company takes a requirement and returns a working product, doing the engineering itself. Price, timescale and how much you must specify all scale across that list.
What each expects from you
The parts shop expects finished files. The builder expects an assembly definition. The hardware company expects a specification and a set of decisions already made about architecture. The development company expects a problem statement and a budget. Arriving with the wrong level of definition is what produces quotes that seem inexplicably high or low.
Intellectual property and who owns what
Settle before work starts who owns the design, the files, any firmware, and any improvement made during the project. Development companies often work to a model where the client owns the output and the supplier retains its own background tools, which is reasonable, but it has to be written down rather than assumed.
How to run the relationship
Define what each build has to demonstrate before ordering it, because a unit for a photograph and a unit for a drop test are assembled differently. Keep the number of open changes small during a build. And ask for the manufacturability comments in writing, because they are the part of the service that is easiest to give verbally and forget.
Questions people ask about hardware prototyping companies
What is the difference between a prototype builder and a development company?
A builder makes and assembles what you specify. A development company does the engineering as well, returning a working unit from a requirement rather than from files.
How much does hardware prototyping cost?
It ranges from the price of a few printed parts to a full engineering programme. The variable is how much of the work you are keeping in house, not the parts themselves.
Who owns the design at the end?
Whatever the contract says. Settle it before the work starts, including files, firmware and improvements made during the project.
How do I choose between similar suppliers?
Ask each one what they would change about the design. The quality of the unsolicited answer separates them faster than any comparison of rates.