Buy boosts when you have the cash to burn (you WILL have cash to burn). If you're "saving up" for anything under 1 million, it's way too early to think about boosts.
Boosts are of very limited use and force your play in a very uncomfortable way; suddenly to max your coin earnings you have to segregate your dragons onto islands based on type. It's just not fun, and the 20% boost is pretty dubious.
Also remember these boosts negatively impact the opposing element. You'll need multiple islands to shuffle dragons around so that you don't significantly limit some dragon types.
The actual benefit is often invisible unless you play extremely actively; you're probably often coming to the game with your coin reserves full/close to full for many habitats already, the boost really only helps if you collect coins so fast your habitats never hit cap.
Generally, Boosts suck. Don't buy them unless you have cash to burn and you're really hardcore about getting the most money. If you do get boosts, get Earth and Cold, and segregate your Earth and Cold dragons habitats onto the same island. With large Cold and Large Earth habitats you can see a real return on your investment. The other elements, not so much.
A special case however is the Pan Long dragon; due to it's elements and high coin rate, it benefits substantially from boosts and can be used to quickly fill Earth Habitats with money. With the Fire and Water boosts Pan Long dragons get a 44% boost to coin rate and can easily get over 400 coins per second at level 15 and over 500 coins at level 20. A habitat full of boosted Pan Long dragons will get cash more than 3x faster than Rainbow dragons, but will require much more attention.