Garbageonly is right, the resources are not valued equally. You see, it is not a good approach to balance things this way. We try to keep cards different, but balanced at the same time.
What you proposed was not a good balancement, becuase all the cards would then have the same effect and same cost, they would just use a different resources. What's the point of having three same types of resources then?
Instead of this, we try to make resources different - each having it's strengths and weaknesses. For example bricks are most easily accumulated, but are also most easily destroyed. Recruits can be used mostly for attacking the enemy and gems are the only resource that permit usage of special cards like "Time warp".