Just don't get in over your head.LOL
I have done a couple of ponds. One roughly 18X14 20 mil UV stabilized liner, granite edgeing, pump, filter, water plants and Koi was 11,000 bucks. Anouther pond I did was 16X12 20mil UV stabilized liner, flagstone edging, no bells or whistles 6500 bucks. The materials that go into edgeing and accessorising the pond really make up the price.
The harder I work
the luckier I get.