In my opinion the yawning portal seems to be one of the most effective buildings for only the cost of 4.
I am considering to use a house rule to balance it a bit more.
My first thought is to increase the building cost to 6.
I'm sure the people who have subscribed to this forum might remember what Yawning Portal does off the top of their head, but ....
You know what? This isn't a very constructive reply I'm writing.
This references yet another thread where the designer explains why they chose the value. I haven't found that one yet, but this might be more helpful.