The Oktoberfest runs from 20/09 to 05/10; some airlines (e.g. easyJet) haven't got their schedules online yet for dates beyond the end of September.
easyJet fly to Munich from Edinburgh and Stansted. The cheapest return fares (at the end of September) start from around �96 (from Stansted, including taxes & charges).
Air Berlin serve Munich from Stansted and Manchester. The cheapest return fare I could find, during the Oktoberfest, was �62 (around �45). That's from Stansted, including t&c. (The basic fare is actually only �1 each way, so you can't get much cheaper!).
Gerrman Wings fly to Munich from Stansted but I can't find any fares to match those of Air Berlin or easyJet.
BA fly from Heathrow to Munich. Their return fares, including t&c, start from �106.
The flights will probably be the cheapest part of your visit. Hotel prices shoot up during the Oktoberfest. My favourite site for finding cheap hotels, Hostelworld.com, can normally find budget hotels for around �20 to �30 per person, per night, in almost any city in the world. However, prices start from around �75pppn during the Oktoberfest. If you're happy with hostel accommodation, though, The Tent (which is exactly what it sounds like) still has a few beds left, on a few nights during the festival, for �14.11 per night.
http://www.hostelworld.com
The official Oktoberfest website might be of interest to you:
http://www.oktoberfest.de/en/index.php
Chris