How to get to the fields?
taxi: Taxi service in Budapest has recently changed: the good part is, all taxis have to use the same fares. The bad part is, prices have gone up quite a bit so airport transfers to the center now cost HUF 8-9000 (EUR 25-30) depending on where exactly you are going. The ride from the airport to the Bubble is around 30 minutes.
airport shuttle: Groups may want to consider the official airport shuttle. A group of 8 can get to the fields for HUF 13500 (EUR 48). You can find their desk at the baggage claim area or in the arrivals hall. Their website.
public transportation: The ceapest but slowest (40-60 minutes) way to get to the city center is by public transportation: take bus no. 100E to the terminus (Deák Ferenc tér) for HUF 900 (EUR 3). To reach the fields from the airport you can hop on to metro line 3 (blue) replacement bus (! due to the reconstruction of the metro) at 100E bus stop Kálvin tér or at its terminus. Here is the scedule of bus 100E.
You can find more information on how to get to and from the Airport on their site.