It must just be a luxury of so many dive ops here in So florida, but Nitrox is not an added expense here, other than maybe the initial EAN cleaning on your gear. I pay $5 for (100cf and 120CF) of whatever mix I want.
As a fairly conservative diver who just likes long bottom times, EAN proves useful in extending those times for me. Not always, but enough to count. I've done Punta sur in cozumel (125-130ft bottom) and logged well over an hour (Okay, it's a multilevel dive, much of whick is at less than the max). But still, my computer (conservative as it is) runs me out of bottom time well before I run out of air (21%) on that dive (And others), when EAN consistently extends these dives to make my gas remaining the limiting factor. It is considerable for me in those circumstances. All I have to do is remain vigilant about not busting my max depth.
As for fatigue and headaches post-dive, I can't really say difinatively. What I can say is that it seems to help more in situations where I have to exert more effort. Even at shallower depths, i seem to recover from sprints faster on EAN than on air.
On those days when we get greedy and go down 4-5x, it absolutely makes a difference. I would get blocked by my (conservative) computer on air trying that.
All that said, reasons NOT to dive EAN :
1- Not properly trained and confident with mixed gas
2- Not sure where my max depth is going to be on a dive relative to my gas %, inculding allowance for possibly having to retrieve someone/something from your planned max.
3- Unable to personally verify EAN % after fill/before dive (NEVER ASSUME the label is right)
4- Equipment not cleaned/rated for EAN
5- Dive is obviously going to be too shallow to make use of the added O2 (If cost matters)
6- Cost and hassle if you're somewhere where either is elevated for using EAN Unless the EAN will tuly matter to the dive.