Quality control is really what it really comes down to. Much of the extra cost comes from QC of some sort. There are layers to QC and each one has a direct relationship with adding cost to the product.
It starts with materials. The truly "top tier" companies are building to some sort of spec that they have developed, has some sort of known margin for error and measures KPIs over time to understand the outcomes as it relates to end user quality. Their spec for a bolt might be for specific materials, hardened, heat treated etc. They have to pay market prices for that part and if they are unable to source at low cost, they will need to pay more to source this inventory or invest in a supply chain to ensure they are able to have the part that meets this specification, otherwise it stops production.
A budget tier may not have this specific of a requirement and will attempt to source low cost parts that may or may not meet the same requirement as the top tier provider.
QC works its way downstream to unit testing, assembly, QA. They may utilize more labor or technology to evaluate if their final product meets their requirements. The same bolt will be magnetic particle inspected, it may be hand fitted and undergo other steps to ensure that it is dimensionally correct, operates correctly etc. It not to say there is a guarantee that everything is perfect, but it simply increases the possibility that when it gets to the end user it will operate as designed and within the durability expectations within the margin of error. They will also closely monitor end user feedback and may even have special programs to continually seek out this feedback so that they can integrate it into the own continuous improvement processes.
Lower tiers are likely not to have as robust QC/QA. That doesn't mean that their products are inherently lower quality or defective, it simply means they won't have as many mechanisms in place to ensure the consistency of their quality.
What every persons has to decide is how valuable is that QC is to you. Are able to seek out the specific qualities of materials that you value and seek out the vendors that deliver on those promises? Are you capable of evaluating the quality of your assembled product, make adjustments or work with the vendor to remedy any issues that you have? If not, are you in the position where you don't put anything at significant risk by trading affordability for a lessor known quantity? Do you want to own QA for this firearm or do you prefer to outsource that to the manufacture at a cost to you? In the end, only you can decide.
