There's actually no official cost for tickets to the Oscars, also known as the Academy Awards. This is because tickets to the awards are sent directly from the Academy of Motion Picture Arts and Sciences to those who have been invited to attend. As it's an invitation-only event, no tickets are sold to anyone.
However, there are several options for those who wish to attend. Celebrities and others who work in the film industry usually get extra tickets for guests they may want to bring along. While it's a long shot for most people, you could try asking someone in the film industry that you know who may have extra tickets.
An option more likely to work for most people is to sign up to be a seat filler at a website like "http://www.seatfiller.com". It's still uncertain that you'll be selected and you'll need to submit a photo, resume, and cover letter, but this will provide you with a seat close to all of the celebrities, even though you're not supposed to speak to them unless spoken to first.
It's extremely difficult to find tickets being sold by a scalper and it's not suggested to do so as it's technically illegal, but tickets have been purchased from scalpers for as much as $30,000 to $40,000. This is also a risk for the buyer, because the Academy has measures in place to insure only those invited can attend. This means that even if you can obtain tickets, you will likely not be able to enter the theater.
While you won't be able to get inside, another option is to sign up for the yearly contest to win a seat in the red-carpet bleachers at the Oscars, which can be done here. Approximately 600 bleacher spots are given away every year.
At the 85th Academy Awards on February 24, 2013, the big winners were Argo for Best Picture, Jennifer Lawrence for Best Actress, Daniel Day-Lewis for Best Actor, Ang Lee for Best Director, Anne Hathaway for Best Supporting Actress, Christoph Waltz for Best Supporting Actor, and Quentin Tarantino for Best Original Screenplay.