f2p games ALWAYS need funding through in-game purchases.
That's not unethical and completely comprehensible.

However, to successfully generate revenue through purchases, people must get the impression that they are getting "their money's worth".

The 2013 Christmas Mystery Boxes, for instance, created a buying frenzy and even I couldn't resist spending 10k gems.
Other than that, the only stuff I bought was limited offer buildings like Floating Residences or Village Schools, because they are irreplacible with ingame cash. Oh, and 2x BK which I thought I bought for Map Fragments but accidentally took the gem option. (aka. misclick)

I would, however, buy more things if the prices wouldn't make me feel like I were getting ripped off.