It's an important concept but it's not marginal utility. Marginal utility is the degree to which adding one more of something will exceed the cost of doing so.
To stick with the motorcycle analogy, suppose I am designing a motorcycle as a manufacturer and I find that it takes 6 bolts to securely fix the engine to the frame. If I use fewer, accidents are very likely to occur, I will get sued, sales will collapse. As it is, accidents will still occur once a year - but that's the industry average (let's say) so I can pay out compensation for that one accident without seeing any real impact on sales. (I have magic litigation insurance so my liability costs are totally predictable for this example.)
Suppose each bolt costs me $10. If I add 8 bolts, the risk of failure drops to zero. I calculate the cost of accident involving my bike, divide it by the number of bikes I expect to sell, and I find it's about $25 per bike; installing those two extra bolts creates $5 of marginal utility or 2.50 each so I go ahead and do it.
Should I not go ahead and add 1 more bolts? No, because it will cost an additional $10 for no noticeable increase in safety, so the marginal utility is -$10.
“Marginal costs” are the additional cost for each additional unit delivered. They can be high or low, but with modern, non-custom software they tend to be extremely low.
That's good, assuming the cost of harm is a cost in utility and not in money, otherwise it starts having issues. Make sure you have a good utility function.
Long-term marginal cost is determined by the nonreusable component cost + minimum refurbish & refit costs per launch. Effectively, the benefits of reuse offer diminising marginal benefit per reuse. Though designing for long-term use (say, a personal automobile vs. a rocketship) can still make sense.
An automobile used for daily commute with a 5 year life sees ~1,000 re-uses. You can vary the parameters to get different results. There are fuel, maintanance, garaging/parking, tax/registration, and insurance costs as well.
Besides, what's important is that the benefit outweighs the cost. If the cost is required to stay in business, then it's probably important enough to incur. Remember: someone's going to be paying that cost and it's clearly worthwhile to them.
Marginal versus average cost. Extremely low marginal cost (cost of the next unit to produce) but extraordinarily high average cost (includes capex). You typically make decisions for profit maximization on marginal cost.
To stick with the motorcycle analogy, suppose I am designing a motorcycle as a manufacturer and I find that it takes 6 bolts to securely fix the engine to the frame. If I use fewer, accidents are very likely to occur, I will get sued, sales will collapse. As it is, accidents will still occur once a year - but that's the industry average (let's say) so I can pay out compensation for that one accident without seeing any real impact on sales. (I have magic litigation insurance so my liability costs are totally predictable for this example.)
Suppose each bolt costs me $10. If I add 8 bolts, the risk of failure drops to zero. I calculate the cost of accident involving my bike, divide it by the number of bikes I expect to sell, and I find it's about $25 per bike; installing those two extra bolts creates $5 of marginal utility or 2.50 each so I go ahead and do it.
Should I not go ahead and add 1 more bolts? No, because it will cost an additional $10 for no noticeable increase in safety, so the marginal utility is -$10.
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