What affects manufacturing costs?

Cost discount falls off quickly with number of engineers, and is capped at 50%. The formula is

Project cost = Base cost * (0.5 + 0.5 * (Minimum engineers / Available engineers)) That is,

• with double the required engineers you get 25% discount,
• with four times the required engineers you get 37.5% discount,
• with infinity engineers you get 50% discount.

Also, there is a 7% rebate for adjacent workshops, but AFAIK, that is returned after the project completes.

The workshop has an adjacency bonus of reducing manufacturing costs if they are next to another workshop.

Also, various continents have full coverage bonuses that can further reduce manufacturing costs.

Finally, the more engineers you have the cheaper it is to produce items.