Why is he being shafted?
Lets be fair even if the build comes in at 15k and the builder makes some profit, who are you to argue that he shouldn't. Why do you consistently infer that all builders are trying to scam people. We are in a business, part of the business is to make a profit. If you don't like it, keep shopping around until you find quotes you are happy with.
You don't begrudge Aldi their profit on the beans you buy, or Virgin the profit on their internet connection they give you - no doubt your current employer also likes to make a profit on the services you provide and you go along with that.
18k was the OP's lowest price, its likely to be the lowest quality.
You wouldn't expect a Bentley at Mondeo prices, certain builders cost more, because they do more. While I agree that a garage is pretty basic, a decent firm will still have the same overheads regardless of the job itself. They are likely to be easier to communicate with, have more tolerance in their price to cope with your ever changing minds, and be clean and tidy, because thats the way they work.
The garage I built at 20k - I priced it as I saw it. It was verified by an independent Quantity Surveyor - so weather you like it or not, some things are more expensive to build than you would imagine. Thats why we professionals price jobs not you.
Back to the OP - if you feel comfortable with the chap at 18k, it would appear he was competitive in his pricing, reference him and make sure the quality is right. If it is, engage him. If not keep shopping around until you can find someone who fits your requirements in terms of quality and price. You are lucky most of us don;t charge for estimation services.
