Also to add to that parts are usually only part of a project. Labor is usually a decent amount of the cost. Plus some profit (usually 2x whatever your parts cost are is a good rule of thumb, until it isnt). You also have to value your own time at this. You will sell at cost if you value your time as 0 or being nice to someone. A 3d print of something is also not something you can necessarily just take to someone and say 'make me 300 of these'. You usually have to work with them a bit and the machinery to make that can cost a bit. There is a weird spot in making things where making a dozen does not cost much other than time and parts. But you want to make 1000 and suddenly the cost goes pear shaped and does not calm down until MR=MC. And you may end up with one custom part you had to make to make the other thing that costs a bunch. Linus tech tips recently had a good summary of what they had to do just to make a screwdriver. You in essence have to build a machine to build your thing. That machine can cost a decent amount.