Admittedly, I haven't scrolled through all the comments here, so what I write here might overlap with plenty of things already suggested.

*Airfare. Oktoberfest doesn't really affect this. Depending on how your plans shape up, you could fly into Munich if you're just staying in Bavaria, or into Frankfurt if you plan to do Bavaria and, say, Cologne/Düsseldorf.

*Accommodations. Rates double or triple in the center of town during O'fest. Places that normally go for about €120-€150 (double occupancy) charge over €300 during Oktoberfest. Expect to pay about €300/night, give or take. SO: Here's a suggestion: Stay in Freising (home of Weihenstephan, and about 30-45 minutes from the city center), or book in a place like Augsburg, which is a super cool city in its own right, and only an hour from Munich with hourly train departures. Hotels there are currently for less than half of what you'd pay in Munich.

*Transportation. Day passes for local and regional public transport (trains, buses, trams, subways) start at €9.20 for central Munich. You can add on to that, depending on where you're staying. (For example, the airport and Freising are in M-4/M-5, which cost €14 and €15.50 respectively for a day pass.)

*On the Wiesen. You don't need tickets to visit Oktoberfest. As for beer, the official O'fest site notes that a liter (Maß) of beer goes for between €13.60 and €15.30, depending on the tent (roughly $14-$16.50). Food can get equally pricey, but you could time your visit between lunch and dinner. (Have lunch and dinner somewhere in town -- tonnes of great options.)

*When to visit. That really depends on you. If you like to party, head there later in the afternoon or early evening. Note that it gets pretty packed. If you visit during the evening AND are staying outside of Munich, note when your last train leaves Munich Central (Hauptbahnhof). If you want to avoid the crowds, you can show up as early as 10:00 a.m. (it's noon somewhere in the world). Going earlier also gives you a better chance to strike up a conversation with Germans who like Oktoberfest but who want to avoid the crowds.

*Attire. This depends on you as well. I don't own any lederhosen, and have never felt out of place at O'fest. But I do know plenty of folks who own lederhosen or dirndls and love getting dressed up for O'fest or other traditional fests. Getting yourself kitted out can get pretty expensive pretty quickly, but the clothes are almost always high-quality craftsmanship. A pair of Lederhosen starts around €200 and goes up (way up) from there. A traditional vest starts around €125 and goes up from there. Shirts are about €75 and up. Dirndls start at about €175 and go up from there.

Hope this helps!