At least for me, there’s relatively few variables to consider – it’s the cost of obtaining the part, factoring in shipping costs and taxes, and adding my desired margin. Factors that influence the margin could be how long it takes you to sit down and do your regular parts ordering, the volume you sell etc.
When it comes to hardware, consider that by you supplying the parts, your customer is receiving several benefits vs them supplying the parts:
– They gain the benefit of your expertise in knowing what parts will fit and what won’t. This reduces the risk of them wasting money on the wrong part, and saves them the time and effort of doing the research and working out what they need.
– If there’s some issue with the part, generally the supplier (you) will take the lead in contacting the vendor/manufacturer and arranging a replacement.
– The supplier (you) handles chasing up shipping delays, lost packages etc
You deserve some compensation for providing these benefits, so don’t short change yourself!
I generally find I can maintain a margin of at least 30% on parts and still keep the price fair for the customer. For parts I can obtain wholesale, this means I can generally supply them at MSRP.
In terms of strings, I charge MSRP. I’m not going to try and compete on retail price with some online megastore that relies on volume and is making margin of 0.5% per set. I’ve never had a customer object to the price I charge for strings – my workshop is not near any retail music stores, so it’s a major convenience for them to not have to make a separate trip or detour to grab strings. The margin for strings varies based on the manufacturers wholesale price structure.