Adam
Have you tried EasyJet from either Gatwick or Luton? Should get the return cost to budapest down from £500 plus a round trip to £200 a round trip.
You have expensive tastes at £100 for a hotel B&B £30 or a Premier at around £60.
If you rent something like a Cirrus 20 you would be looking at about £2300 for the aircraft or £2500 all in.
What you are saving by flying is time and hassle
Pace