Yeah honestly the biggest fail on their part was fucking up the original quote despite being the supposed experts in well drilling.

So basically they gave you a quote which was 50% too low because of this fuckup, got you to agree to it and give a deposit, and they don't want to let you out of it because the job is far more than they originally quoted due to their own oversight. I can't see that standing up in court.

You might even have a case regarding forcing them to do the job for $5400 instead of $11k, but you probably don't want to compel them to the job while resenting you. But LOL at thinking they can keep their deposit over this.