It is usually much cheaper to just put in a concrete one. Dig a 6' deep hole, pour a slab with drains, build concrete block walls, then finish it with plaster. Seal with epoxy sealer, paint with epoxy paint, and voila! I built a 6000 gallon one by hand. Took me an entire summer, but 22 years later it is still going strong. Maintenance costs are low - had to replace a pump once, and that's it.
Like so much with construction - do it right at the beginning and it lasts forever. Round corners at the beginning, and you find yourself paying for it over the long haul.
I agree 100% with that, as an engineer I apply and/or encourage (sometimes successfully) that logic almost daily.
However, to each his own when it comes to recreation/hobby (which for me this is).
I am someone who likes to tinker and enjoys the discovery process (even if I am often reinventing the wheel) and to be able to be proud of a successful result (and all too often own the failure but happy in the learning that occurred). Could I have built a better pond, probably faster, and cheaper? Yes, there is little doubt (please don't tell my wife). But what fun is that for someone like me? You should see the greenhouse controller that I designed and built myself out of a single board computer (raspberry pi), micro processor (Arduino), relays, irrigation valve (pump in the winter) and space heater (it even has its own web site!). Could I have purchased something that would do the job (probably better and/or cheaper)? Would I probably not have lost those plants that died due to failures while I worked out problems with the system? Yes to both. Do I regret doing what I did? Not a chance (well I kind of regret killing a couple of nice trees). I have a system that while it is probably not suitable for anyone else in the world, is 100% my own creation and I am proud of it (and get to tinker with it constantly).
It is also why I have about six Eastern White Pines (ordered by accident). Are they hard if not impossible to make into bonsai? Yes. Are they way above my knowledge/skill set to make into bonsai? With out a doubt. Will they ever be "bonsai"? I can say with 99.9% certainly, NO. Do I have fun experimenting with them and take eminence pride in the few wins I achieve, not to mention what I learn? A great big YES.
Yikes, that started out as a simple one line reply but took on a life of its own quickly. I probably should not have had that second espresso this morning. If you got this far you are probably realizing you should have stopped after reading the first line and skipped my entire rant. Sorry. ;-D