It comes down to the marginal utility of your money vs. the upgrade.
For example, I fly Singapore to Tokyo about once or twice a year, sometimes more often, always on Scoot, a budget airline run by Singapore Airlines. Upgrading from economy to ScootBiz costs as little as $80 with their "bid for biz" program. I always "bid for biz" and it's absolutely worth it: early boarding, free food, much more space (for yourself and your carry on), etc. So the marginal utility of the upgrade for me is worth more than $80.
At some level of net worth, flying First on a legacy airline to Tokyo, maybe Japan Airlines which I had to take in economy once for $1,500 vs the $200 or so Scoot can go down to, would have more utility to me than the $10,000 extra I'd have to fork out. I don't know what that net worth would be (probably north of 50 million), but I know that past perhaps a few million dollars in liquid assets, I would always fly business even at 3x the cost, and I know plenty of people in that situation doing exactly that. And there's people for whom flying SQ Suites [1] for $30,000 one way is a "discount" from flying the same route privately.
One issue is what your time is worth. Few of us can travel long distances in Coach (say, Phoenix to Europe) without losing at least one day to physical recovery. If I can arrive rested and ready to get to work, that's worth money.
Return flights are usually something like:
* economy 900USD
* premium 2000USD
* business 6,000USD
* first 10,000USD
So, I always go economy.
Not sure it's ever worth a month's wages (after tax) to have a bit of free champagne.