San Juan variant to balance prices
Have you played with the San Juan expansion buildings from the Treasure Chest set?  While I agree that there are some cards that are simply better than others, the expansion tries to fix this.

For example, Park is a 3-cost building that gives you a discount of 6 instead of 3 if you overbuild it with a Crane, which might make the crane more enticing.

The Residence allows you to score points extra points for each set of 3 buildings with the same building cost.  This could convince you to build some of the less-desired buildings to get the bonus.

And personally, I like the Carpenter -- I've had a few games where I bought a ton of purple buildings, and was able to refill my hand quickly.  It could have been a few lucky draws, but the Carpenter is at least decent in my opinion.