Hello @Erick Setubal there are more areas where you can save significant amount of money. For example:
- PCB
if you go for very high volumes, I would recommend to discuss your PCB with PCB manufacturer. They can recommend some design changes which can optimize PCB manufacturing process and make the PCB cheaper (e.g. panel size optimization, V-Scoring vs Milling, VIA size, Minimum recommended track/gap, using blind and buried VIAs, PCB material, number of layers, PCB color, .....).
- Schematic
About components and schematic - that may be sometimes tricky. For example sometimes it is very hard to say how many decoupling capacitors and what values is still good enough to have reliable and working design. Also, normally I try to use as few different components as possible (e.g. invertor can be replaced by NAND, 100nF/16V capacitor by 100nF/50V - as you already mentioned) so we can buy them in higher quantity and that means cheaper. Of course, watch the price of the most expensive components and choose the right one. What you can do - is to make a working design first, and then optimize it. We also often create different Variants of one board - a cheap one, middle one and most expensive one.
- Replacement & Suppliers
A lot of cost optimization can be done by finding a cheaper replacement (not directly changing schematic, but checking suppliers) - e.g. capacitors are good example. Sometimes you can buy one for $1, but when you find the right value / voltage / manufacturer, you can safe half of the price.
- Assembly house
There are sometimes big differences in price for assembly. Again, after you run the first prototype manufacturing, ask your assembly house for recommendations to optimize the assembly process. Some of the recommendations are: not placing components close to the PCB edge, not placing SMT close to through hole pins, ..... The assembly house will exactly tell you what kind of problems they had during manufacturing and what took a lot of time to do it or fix it.
- Testing
Design a simple and quick production test - time is money
Probably, the most helpful would be a list of recommendations from PCB manufacturer, Assembly house, ... if you find some, please let me know. I would like to have one too. Thank you.