Well sir, marginal utility is great. But should we go that far when the only thing that begs answering is how fast will you repay for buzzard while grinding?
Yes, we should. Let's say Bob and Joe both have exactly 1.75 million in the bank. Bob buys a buzzard, and Joe doesn't. They then both play Chopper Tail 100 times (and in this toy model, do nothing else; no additional expenditures etc etc).
After 100 Chopper Tails:
Bob's money => $0 + (100 * $17,500) = $1,750,000
Joe's money => $1,750,000 + (100 * $17,500)= $3,500,000
But this is misleading. We don't want to know how much money they have after the same amount of missions; we want to know how much money they both have after the same amount of time. Otherwise we could just say "Hey I bought a Super Diamond for doing Chopper Tail and it's paid for itself in 15 missions!".
Let's now say that it takes Bob three minutes to complete Chopper Tail in a Buzzard, and Joe 5 mins without a Buzzard, and compare those figures.
Bob's mission/hr is 20 (60 mins divided by 3 mins).
Joe's mission/hr is 12 (60 mins divided by 5 mins).
So in one hour:
Bob makes $17,500 * 20 = $350,000
Joe makes $17,500 * 12 = $210,000
So buying the Buzzard makes Bob and extra $140,000 per hour. If these figures are correct*, Bob's Buzzard will have truly paid for itself after $1,750,000 / $140,000 = 12 hours of solid grinding.
* They aren't correct, obviously, due loading times, the fact I've guessimated how much faster it is in a Buzzard etc etc etc.
Theoretically, we should take into account payouts for different missions minus delivery expenditures. Thing is, it's still a theoretical approach. In reality new player may spend a lot of time practising Buzzard and there is no guarantee that he will be faster with mission while using it.
And stil nice to see educated people here like you.
Yeah there's lots of other stuff we could put in our simplified model, but my version is closer to an accurate model than the first. And thanks!