We're building a new house and our builder had allocated $2000 for a retaining wall needed along one of our boundaries. It's now built and it actually cost $25,000!
You need to start talking to a lawyer as it should not have increased 1250% without you being informed of the increase or potential increase.
I don't see any good options. Even if you talk to a lawyer and the builder has to wear it - or some of it - do you really want your builder offside and trying to "make up losses" when so much work remains to be done?
Like RV, to me the really unconscionable thing is being hit with such a huge variation after the work has been done, when your options are severely limited. Surely the builder must have told the contractor who did the retaining wall that there was a $2K provision, and if so, the contractor must have immediately started laughing. Why wasn't it addressed at this point? If the contractor had quoted $25K, you could at least have had the option of getting other quotes, or even electing to do the work yourselves (if you're so inclined), or go back to a timber wall. It really is outrageous to have a provision of $2K and hit you with a $25K bill with no warning. And this stuff about draftsman's drawings is just ridiculous. At the point when you got a $25K quote that argument may have held some sway, but there came a point when the retaining wall guy turned up and saw what the job was, and knew that $2K wasn't going to cut it, and that's when it should have been addressed.
I think I would express to the builder, in writing (or email - anything that can be used as proof later if necessary), that you're extremely dissatisfied with being hit with such a huge variation without any warning. Tell him that you seek his agreement that in future, he will advise you of any variations more than 20% (or whatever) from the provision, or $200 (whichever is the greater), prior to the work being commenced or the item purchased. If he's confident of his provisions, there's no reason why he shouldn't agree to this.
I'm sorry you've had such an expensive lesson, and that the remainder of the build goes more smoothly.