Are you wanting to approach this commercially, or is it just for friends and acquaintances? Maybe come out with a decent number like 50-100% of your filament cost or possibly more for this kind of thing to account for your time in setting things up, any design work, etc.
Are you doing it commercially?
You need to have some idea of cost, the more accurate way to do it would be to know the lifespan of all of the parts in your machine or set a replacement schedule for them, break down the cost of the component per hour but that’s hard to get numbers on. Try to develop a maintenance schedule for the printer to replace common wear parts such as nozzles, bearings, etc. every x hours, then determine the cost per hour to achieve that maintenance schedule and account for a little overhead for unexpected failures (electronics, etc). If a part takes longer to fail, then fine, but at least you achieve your minimal maintenance costs.
How much is your personal time worth?
Are you designing objects for them, or just plating and printing them, if it’s the former and your time is valuable to you, you may need to figure out a rate for that as well if it’s the latter than it’s not worth as much.
Power consumption as well?
You have the power required to run the printer, but you also need the lighting, and heating/cooling required to keep within reasonably stable temps for printing.
Don’t forget things such as packaging materials, shipping costs (supplies to you, parts out to them), tax liabilities, fuel to go to the shipping place, money out for stock/inventory, etc. A lot of these are pass through costs but sometimes you can get stuck holding the bag on expenses you didn’t expect it, or you didn’t actually pass it through for some reason.
If approaching it like commercial business you really need to fire up a spreadsheet and get to work breaking down all conceivable costs and income sources and see if you can do it without ‘losing your shirt’ while remaining competitive to other options these people may have. If it’s just making parts for friends and acquaintances + making a little bit back then you can be more free willed.
Do you want to make back the money you spent on the printer or do you just want to repair and upgrade it? If you want to make it back then add that into your costs as well.