this is basically the start point and at relatively close scales will work pretty well.
this can also be expressed as a 2 in 10 (20%) bulk discount.
you can add on adjustments if you can estimate other factors like the admin cost (of your time) per transaction, a defect rate cost or spoilage, cost of storage and transportation and so on.
These overheads can sometimes be shared across other products, especially transport and storage.
for longer term stuff you might want to translate this into an equivalent annual (or longer ) cost per year/day of consumption that covers expected consumption with your safety margin.
if you need to account for every element of you cost base to the nearest penny then it can get pretty complicated - but if you're prepared to make simplifying assumptions like:
"It doesn't spoil over time",
"I already have some unused space",
"i don't need insurance on storage.",
"i'm going to ignore any adjustments lower than 0.001 cents per unit (a materiality threshold)"
Whether those assumptions are a good idea or not, depends upon the wider context.
Except buying objects in low quantities allow defects to be detected much sooner, letting you exchange/return/work a deal with the seller. If you buy stuff in bulk and realise the fourth sack of flour has mold, you're likely to already have passed the reasonable threshold for exchange or return.
Okay. Well is the item a food product or a tech product?
Is it insured? What's the cost to ship it back and get an exchange or refund if it's a defective unit? Who pays to ship it back if so? How much does it cost to keep in storage until it sells?
There are way more questions that need answers to even halfway try answering OP. Bananas need refrigeration, fidget spinners need little more than a box.
OP didn't say what they were looking to order in bulk, but doing simple arithmetic without sufficient information means basocally squat.
You misunderstand. Assuming you're buying 200 of product A or 750 of product A, you should expect the same proportion to be defective. Nobody's suggesting two entirely separate products should have the same failure rate, but the question is also not about comparing two entirely separate products
You totally missed my point. OP didn't even distinguish whether they're looking to purchase produce (which has a short shelf life), or looking to purchase electronics or mechanical devices (which tend to have a fairly long shelf life).
It costs either way to keep items on the shelf. But, like, what's it cost to keep the items on the shelf long enough to sell them without half your stock rotting away?