What is the process and cost of finding a manufacturer?

Hi, I am finished doing the paperwork for setting up my LLC, trademark, provisional patent, related paperwork. I am now searching for a manufacturer. How much does one usually cost, and what is the process of finding one? I am getting mixed information from a sourcing company, chinese manufacturing, and a USA manufacturer. The sourcing company says I need 3D drawings first to get manufacturing, the chinese can just build a prototype (low quality) from rough sketches, and the USA manufacturer says it requires engineers and $10,000 of prototyping before manufacturing. Who is right? My product is very simple, just a plastic case with a magnet inside and a clear cover. Does anyone have experience getting their invention manufactured, and is there anything I need to look out for? (Also, any good manufacturers in the USA that you have used, or know about?)