I think wagers are decent as it is, considering that you need a clan shard and a large siege stone making a city 60k+wager and if HD is 67500 most places should be 80-90k or more pushing the cost to just DROP a siege to 140-150k. I think that's high enough IMO. You then have to factor in costs for the siege equipment to take down walls and ultimately the clan stone. Warhulks/ships make this cheaper as powder is dirt cheap but you have to risk 25-100k in siege machinery still to do this.
Walls need to come down in health or siege equipment needs to be buffed/cheapened, walls are just annoying to take down; IMO should be invulnerable until like 4hrs before clanstone goes vulnerable or something so that a clan has to actually kill them while the enemy is there to defend, then make siege equipment do more structure damage, lower wall health, or make siege equipment cheaper/more effective such as allowing cannons to have unlimited shots and cut their health down to very little so they can be destroyed easily.