This was also asked on Dinonline and answered by them :
Question: A nice Jewish boy sees a mistake online to purchase tickets to Israel from a Jewish company (ELAL). It gets posted on a forum and within hours thousands of tickets are purchased at the mistaken price before the company is able to correct the mistake. The company is probably going to lose hundreds of thousands if not millions of dollars from this mistake.
Halachik points to ponder:
A) It is a mistaken fare can I go L’chatchila and purchase this fare? Is there a Halachik difference if I am purchasing it from a Non Jewish middleman (Expedia), or from the Jewish company directly.
B) There are DOT and FAA rules that specifically discuss price mistakes,and state that even an egregious error must be honored or face fines of up to $27,000 per occurrence. What is the Halachik viewpoint on this, does this make this transaction fall under the rules of Dina D’malchusa?
C) Elal has announced that it will honor the tickets purchased and it would seem(?) that for now it is permissible to use the ticket, as they are making a business decision to honor the mistake rather than risk business fallout and/or potential fines. What if they were to deny the tickets and cancel all of them. Can I complain to the DOT to compel them to honor them or to receive compensation?
D) I am the first one to notice this mistake, am I allowed to post this deal to allow others to jump in? What about the Chiyuv of “Hashavas Aveidah”, and “Lo Sa’amod al Dam”. I know that he is taking a financial hit from this am I obligated to inform him? Am I allowed to purchase it for myself? Can I tell my friends if it is already public knowledge?
E) What are the the Halachk ramifications of “Onah” price mistakes in excess of 1/6?
Answer: The final point you mention is the most important. The mistake in the price of the tickets was clearly far more than the ona’ah rate of 1/6, and therefore it would not be permitted to make the most of it, and purchase tickets from a Jewish seller at the mistaken rate. This halachah does not apply to a non-Jewish seller.
In the same vein, it would not be permitted to “spread the word” and cause others to take advantage of the mistake.
The halachah applies in spite of the fact that the secular law obligates them to honor the contract. This will be of possible interest bedi’eved, but not on a lechatchilah level. Even for less that 1/6 difference in price, where the sale cannot be voided or the ona’ah collected, it is forbidden to exploit mistakes (this point is subject to a dispute among rishonim; see Rosh on Perek Ha-Zahav).
However, there is room to argue that an airline ticket is “ein gufo mamon,” meaning that it is not actual money (its value is only its potential to allow a person into the plane and on the flight) and therefore the laws of ona’ah will not apply here.
Nonetheless, because the airline certainly does not mean to sell tickets for this price, and the entire sale is a mistake which was presumably caused by a computer error, it follows that in Torah law the sale does not stand, and it is not permitted to take advantage of this mistake.
Bedieved, if El Al state that they forego the mistake, a purchased ticket can be used. Although the law forces them to forego the mistake, this will not be considered talyuhu ve-yahiv — it cannot be compared to a gunman who forces somebody to forego his money at gunpoint, because every airline trades with the knowledge that in cases of mistakes it will have to forego the mistake, and when the airline begins trading it does so with intention of foregoing such mistakes should they happen, so that bedieved the airline ticket can be used.