There is no way to forecast the costs accurately. If your decision depends on it, then be careful!
Annuals will be between 10k euros and 30k euros
Insurance is fairly cheap - relates to hull value, say 5k euros
Avgas burn is 130-160l/hr
Eurocontrol charges are ~50euro/hr
Engines are about 50k euro to overhaul, each
I have two bits of advice. Invest about 0.2% of what a 421 will cost to join
Cessna Pilots Association. The Twin Cessna forum is a vast archive of knowledge. You could spend days reading about every nuance of 421 ownership and operations. Also, a typical pre-purchase costing say 1000euro will review all the logs and paperwork and involve a reasonable but somewhat cursory inspection. It will not discover problems that could cost a very significant proportion of the purchase price to fix. Pay for something closer to an annual inspection (including inspecting inside the cylinders etc for corrosion). It will be a tough decision - because you could invest 2k-3k euros in inspection and test flight and than have to walk away. Better you do that a few times than buy the wrong airplane.
brgds
421C