Estimate on price - building a pond

I live in Maryland near Washington, D.C.. Let's say I wanted to build
a pond outside for koi and water plants. I'd be willing to do all of
the work myself, soo ... how much would it cost? I know this depends
on how big it's going to be, so let's say big enough so that it doesn't
freeze over the winter (to keep the fish alive). If that's even
possible, anyway. Presumably I'd need some sort of lining material, the
fish, the plants, and a pump. How much would I expect to pay? And
there's probably other stuff I haven't thought of, seeing as how I
haven't done this before.