Printed circuit board to be needed

I have sent some PCB designs to factories for prototyping. The price, however, varies greatly. Some ask 200USD for a four-layer 5cm*10cm PCB prototyping. While some only ask for 50USD. I assume that there are differences between these two factories. However, I could not find the differences. Could I ask for some advice here?

One thing that can raise the cost difference is electrical test: All or some

Prototyping is a hit and miss thing.... Some houses do more than others... That way they can spread the costs between several customers.... Other houses bundle the whole PCB costs onto one customer!! They still need to fork out for tooling and setting up... If three other customers require the same type of board as you, then the tooling cost will be split 4 ways...

There is a huge difference in costs between boards made in tgmhe US (and I'm guessing Europe too) and China. I have been using Chinese fab houses with good results at a small fraction of what it would cost to build the boards in the US.