the only thing with koi is that financially it can be pretty crippling
we have got about £2000/£2500 worth of fish in our pond and me dad has been thinking about filling it in (hard to maintain it on his own without any help) and the most that anyone has offered for the fish is £250, he's decided now that he is keeping them (much to my delight)
its awesome though from when you start building it just watching it take shape and then getting the plant life around it to look just how you want it, then when you throw in some big bastard fish and get them used to you so they swim over to you and eat out of your hand, the best bit though is going out and buying some proper baby fish as cheap as you can and watching them grow into monsters.
you have to educate yourself quite a bit as well to learn how to get the water just right for them, I found myself just reading up on the different breeds of them, I can name now pretty much every different type of koi carp