For organization of power selection?
Generally, when I build, I build for exempability.
Now, if you spend the entirety of your at L50 (like a farmer), it doesn't really matter.
As for how you slot enhancements, that's almost an entirely personal preference.
Yes, maxing out a build for damage, survivability, etc is expensive.
As to cost effective? Well, depends on how easily you can accumulate wealth.
Currently I'm sitting on just over 3B in Inf, 600 Empies, 240 Hero Merits, 10,000+ Reward Merits and around 6,000 Vanguard merits. Plus an unspecified number of Architect tickets.