How to estimate volume costs in hardware project?

I'm working on a hardware project, and I am deciding on several avenues of hardware. My question is, how do I evaluate each avenue? I assume my hardware will be assembled in China for cost purposes, but the only places I know to get quotes for hardware are avnet express, digikey, arrow, etc. Large distributors.

Not to mention, image sensors are incredibly difficult to source. Long range wireless radios are a bit easier, but for me still difficult.

Does HN have any advice?