Who needs to buy a yacht when you can own the Batwing! Frankly, the bulk of the millions spent is on that self-flying piece of awesomeness, with the closest thing in real life being the US Marine V-22 Osprey which runs for about $60 million. Cut that out and you have a very reasonable 20 million dollar superhero.
Still, a super-hero knows where to pinch pennies. It's nice to see that Bruce Wayne makes his own DIY Batarangs. That doesn't cost him more than $45 in sheet metal.
Source: MovieClips, Gizmodo