Belgian beers really vary. I'm not sure why you say many Belgian-style beers taste like banana or are rubbery. Actually I find most of the Belgian-style beers that aren't from Belgium weak and lacking in flavor and aroma.
I tend to prefer the darker Belgians (dubbels and quads) to the tripels and there are a few good American ones. Ommegang Abbey Ale and Three Philosophers, Pretty Things Baby Tree, and Schlafly Quadrupel are a few of my favorites. They can't compare to those coming from Belgium, though, namely Rochefort 10, Westvleteren 12, and De Struise Pannepot. Those are beers with wonderful complex aromas and flavors that no American Belgian-style beer can come close to approximating.
Belgians are expensive for a homebrewer to make:
1) You need a Belgian yeast. This means using a liquid yeast from wyeast or white labs unless you are using safbrew T-58.
2) They are generally strong beers and strong beers mean more malt and therefore a higher cost.
3) Belgian candi syrup must be imported from Belgium. There are presently no American producers, and no, you can not make it yourself. It can get rather expensive ($6/lb). You can't make the best Belgian-style beers without it and I think that is a big reason why the American belgian-style beers are so generally so poor.
As for why commercial Belgian-style beers are expensive it's because they are a specialty beer, generally are more difficult to make, and are usually aged and may contain additional ingredients such as fruit or spices (dried plums in Baby Tree, spices in Pannepot, Kriek in Three Philosophers).