I have a 2000 gallon koi pond. If I won the powerball it would quickly become a shark pond. Because I'm in Utah, it would cost a lot to heat and I would have to build a structure over it so that it wouldn't waste too much energy.
If I had the space I would build a wood frame, line it with carpet, and put a pond liner in it. I bet it would be less than $400 to setup and it would be about 1000 gallons. Then down the road if I had the money for a better tank, this build could be converted to the sump.
I think this is very cool and I'll keep watching and seeing how it goes for you as I dream of something like this in the future.