gzt: Imho people just make themselves look silly when they insist on taking advantage of web pricing mistakes/errors.
Even so it is a good question when purchase has occurred and it will have to be resolved one way or the other at some point. IIRC delivery is part of the contract and because delivery has not occurred in this case and the party has been notified of the error and the error was obvious that is where it will end.
Not sure entirely when the 'genuine error' back-out point expires, but legally a contract is composed of an offer, and acceptance and consideration. They advertised the PC, which is not an offer, just an invitation to make an offer. He made the offer by placing his order, they accepted the offer by accepting his order, he supplied his part of the consideration by giving them money, their part of the consideration is a promise to deliver the goods within a given timeframe.
Legally there's no requirement for the contract to be fair or reasonable - the amount he pays does not invalidate the contract. The Sale of Goods Act allows for a genuine error in an advertisement, but I'm not sure if it gets as far as a contract being completed that genuine error is atill applicable.
Guess who is halfway through a small business course, and doing his assessment for the legal module?