when i looked up the wiki info on the unicorn rift frequency, it got me thinking how much this affects unicorn generation, so i did some math and came up with an equation everyone can use, to calculate when it would make more sense to buy a tower instead of other zigurat buildings. be aware though, this doesn't take ivory or alicorns into account and since unicorn rifts are random, isn't 100% reliable either. so here goes:

2500/A 10=B
C/B*D=E

the first line is to calculate the total boost an ivory tower will give you (rifts pasture boost), the second line is to calculate when to buy the next building.

A is the amount of unicorn pastures you've built
B is the total boost (in percent)
C is the current amount of tears needed to upgrade the building you're using for comparison
D is the pasture boost % stated on that building (5% for tomb, 25% for citadel etc.)
E is the price limit for the next ivory tower upgrade, which is to say, if another ivory tower costs more than E, buy the other building instead.

if you need clarification on something, ask away.
cheers