Once you get to the point where you can buy the "Fountain of Youth" cultural building... it's 700 or 800 diamonds for a 3x3 culture building but also adds 700 population. So, when you consider the fact that 4x3 humans lvl 15 residence is only 560... it saves you squares and gives you culture.
Considering most space is used on residences... it's a cheaper option than buying a space expansion... gives you the population immediately and bonus you get culture.
There is an analysis I did in my Facebook group (Elvenar Platinum Leaf) comparing 'Hybrid' buildings (that give pop + culture combined) with using the same number of squares to buy some pop buildings and some culture buildings, separately.
It is Tip # 66.
Most 'hybrid' buildings are poor buys.
There ARE exceptions, particularly high up the Tech tree - like Snail Palaces which, at $25 a pop, give 2.5 times more pop/culture per block.
But if your Daddy doesn't own a credit card company - particularly for lower level players -
don't buy buildings for diamonds!
Why?
Because as you progress in the game, bigger and better buildings are unlocked - and in a month or two you will be left holding a lot of worthless expensive tiddlywink buildings that you will have to discard.
So what I tell my 'noobs' is, if you want to use Diamonds to advance your game:
1. Buy extra builders. They do not lose their value as time goes by. They are
the best diamond buy; they speed up your game and make it more exciting!
2. Buy city expansions - for the same reason. You get 25 extra blocks in your city that never lose value - if the buildings you place on them get old and tacky and obsolete, you can sell them - but you do not lose the expansion...