I just purchased enough to get 3rd builders in two cities....then event started and got wishing well to generate diamonds.
I used some earned diamonds to purchase 1 lv up for one phoenix in one city, was at stage 9, now stage 10.
I just left the other phoenix in stage 9 because that would mean more $ to spend to get essence to get evolving artifact...
if you don't plan on spending real money, I say wait for event, get a wishing well and just use diamond earned to buy builder.
3 builders are enough I think....
well in game, as you play and be patient, you would eventually get some free diamonds to cover more builders, expansions or magic academy.
i'm fine with lv 2 magic academy, got lots spells from event so I am not crafting or creating spells that much anyways.
expansions are good but I can get by what I can manage.
it really comes down to your playing style.