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.
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.
[1] reposting because such a fun read: http://dereklow.co/what-its-like-to-fly-the-23000-singapore-...
When I woke up at my destination, significantly less jetlagged and actually rested, I said "now I understand."
I still don't pay for it, but it's certainly worth something.
Even using American's post-devaluation chart (one-way):
* economy 35,000
* business 70,000
* first 90,000
So, buy miles when on sale, fly first at a significant discount subject to availability.
[1] https://www.wolframalpha.com/input/?i=quadratic+fit+%7B1,900... [2] https://www.wolframalpha.com/input/?i=log+fit+%7B1,35000%7D+...