You cannot discuss people making 'good returns' and 'good offers' - the bloke who offered me £50 for a three letter today though he was making a good offer and would be a good return on my outlay to get it. Would I sell? not a chance...
Yep, it's quite literally none of the seller's business what the domain initially cost, as it has no bearing whatsoever on its actual value, which is - to belabour the point - whatever the "market" will pay.
However, the curiously twisted British concept of "fair play" seems to leak fairly frequently into what should be 100% pragmatic business transactions - you're certainly neither the first nor the last person to argue on the spurious basis of a "fair" or a "good" return.
If you were dealing with a US corporation as a prospective buyer, for instance, they would probably be analysing a domain buy from dozens of angles, but all of those angles would revolve around figuring out the exact value of that domain TO THEM and none would have anything to do with what the domain might initially have cost the buyer.
A "usable" piece of land in Knightsbridge such as a store might seek to build on could easily cost £20,000,000 or more, yet they don't begrudge the price that the buyer is seeking based on the fact that at some historical time in the past, the same parcel of land might have been sold to somebody's great great grandfather for £5. The buyer can't say "Well, your family bought that land for £5 so I think £50,000 is a simply stupendous return" - a laughable argument, yet the same one you seek to present to "justify" undervaluing domains.
Incidentally, the choice of amateurdramatics.co.uk as an example is amusing because that specific domain isn't truly for sale (ok, anything is for sale AT SOME PRICE) because my sister probably wants to use it at some point. There are literally thousands of amateur dramatics groups in the UK, but there will only ever be one amateurdramatics.co.uk so there couldn't be a better domain for the resource site she's thinking about building.
Here's the rub: unless you're Household Name company X, practically nobody's going to have heard of you, your products, your services or your brand. So you can short-circuit the whole hyper-expensive "branding" process and go for credibility that money (almost) can't buy by securing THE generic for that particular niche. And that's worth paying for, above and beyond the traffic/income generated by the domain (which is also a factor). More and more companies "get it", so why should sellers be prepared to accept anything less than the going rate for their names?