Upgrade offers don't look at the original price of your ticket. They tend to be decided more on the basis of how many seats might be available to fill (if there are few seats left in business, then they would ask for more; if there are lots of seats left in business, then it is unlikely that they will sell out based on last-minute business travel, so they can afford to offer a few seats at a lower-than-normal rate), what the "usual" cost of a business sector might be, and how long the segment is.

Of course, if you paid a really low fare, then any amount is going to be a much greater percentage than for those people travelling on your flight who might have paid many multiples of your total fare.

I'm not sure why you have decided to multiply the offer amount by 4 - true, you have 4 segments, but you are not being offered an upgrade for all 4, and even if you were, that's not how it works, each sector would be offered separately at possibly different rates. You don't have to upgrade this flight - or even all 4, even if you were to receive an offer for all 4 and were interested.

€640 to upgrade a Europe-Gulf (or vice versa) segment is pretty much standard, across most of the ME3 airlines.

They're obviously not going to pitch the price so low that every passenger in economy class would consider it a bargain. And they're not going to pitch is so high that no passenger in economy class would ever be tempted.

You already once had the option to buy a business-class seat for the flight; for your own reasons you decided against it. You are now being given a second chance for this sector only - you clearly don't find the price attractive, but that does not mean that it is an unreasonable amount, or that QR are insane to ask for such a price.

People do sometimes indeed pay these amounts for upgrades on similar flights - and when a csutomer is happy to pay this, then that's not invalidated by you, or any other passenger, not finding the offer attractive.