Hire a civil engineer and come up with a plan, and then estimate based on that plan. There are dozens of ways to deal with active or expansive soil. If the existing improvements are actually on fill that has settled, there may be no reason to add caissons, just level and seal the existing foundation. If it is still active, the difference between doing it right and doing it wrong could be hundreds of thousands of dollars. The worst thing you could do is rely on some guy on a blog that doesn’t know what he’s talking about.
There is no reason that improvements should cost close to $300 to $400 a foot if you’re not doing anything too wild.
