Hi kevin...
Please, read the thread on top of this forum, named :
Guide to the Ideas section *Please read before posting*
Next time, use the Idea-template found in that thread.
It is deliberately, that you only have a certain amount of expansions available per age... otherwise the game would simply get too easy.
It is meant to be difficult...
Buying more space with easily attained supplies (and/or coins) would simply ruin the game-play.
Your proposal will definitely make it easier to progress through the ages and is thus automatically disqualified... (read the yellow text in the linked thread)
This idea is a "no-go".
Thread closed...