Like every persuit, electronics requires stuff - to make the terms used unambiguous, we can break this stuff down into several categories, which need to be approached differently.
- components are incorporated into the final product in a direct correspondence - these are things like resistors, capacitors, IC's, and so on. Assembling N copies of a design will require N times as many components as one of them, barring any mistakes (like damaging or losing components - which is unavoidable. Manufacuring houses will usually specify a percentage of extra parts that you need to pay for to account for components that are lost during assembly, usually based on the size of the part; loading a reel of passive components into a pick and place usually involves a number of them falling out of the tape, these are almost never worth trying to recover). These have the most direct effect on per unit cost, as well as the performance and behavior of the board.
- modules are components which consist of a number (often a large number) of components already assembled, and ready to place as a single unit. These are particularly common for RF/wirelesss devices, due to both FCC or your national equivalent rules (which penalize anyone hoping to both obey the law and design all aspeacts of their hardware, and favor the use of modules, as well as the importation of unregulated modules from lawless foreign firms - something to think about next time some politician is ranting about how evil china is and how they've crushed domestic western industries: ) and the difficulty of RF design which would otherwise limit use of these devices. A common example is the ESP8266. While you can buy the raw chip, very few people (at least who would be reading this) will design at that level. Instead, a little PCB with castellated pads
- consumables are things that are incorporated into the final product, but not in discrete units (for example solder or hot-melt glue) or which are nominally tools, but which fail and need to be replaced periodically (tweezers, soldering iron tips, that kind of thing). These rarely have a large impact on costs. This gives you more latitude to use more expensive consumables - presuming they make a difference. If using fancier solder paste results in fewer bridges and hence less need for rework, it's almsot certainly in your interest to use that kind instead of cheapo solder paste that you need to do more rework on - except in extreme cases (ex: $50 tubes of solder paste vs $3 ones, where the expensive one provided only minimal benefit). I have two grades of flux, cheap stuff that works well when you just need some flux, and I have Kester RF-741 (about 5-10 times the cost, maybe more), which I use for any "challenging" rework task, but provides no improvement in most tasks.
- Tools are things that you can't assemble something without, but which is not used up, damaged, or worn out, and will last years or decades without becoming worse. Soldering irons, reflow ovens, hot air guns. Decisions on whether to cheap out or spend big are here involve the largest sums of money, and are the hardest sort of decision to make, because the cost is large and immediate - while the benefit is small and long term - but if it does provide a benefit, that may add up to a huge benefit. Or maybe the benefit won't be as large as you hoped.