It's really not expensive, filteration would cost a bit but not alot, the pond build would be really cheap, but as yu said the space you need is the costly bit
unless you had the cash to build an out door fish shed kind of thing you wouldn't build a tropical pond unless you had the space. filtration would be cheaper than on a big tank as you would use a big pond filter. the only costly things would be the heater and a good pump.
soon as i get a house i will be building a big fish house with a pond but i like looking at fish from the side so i would have to put a big glass panel in the front!!