How 'bout this for a variation.
Why not make the base concrete, and the sidewalls out of cinder block?
I'm not experienced enough with this stuff to make a cost comparison, but the liner needs to be around triple the surface area of the pond, but concrete would only need to be exactly the surface area. The liner comes in at around 70 cents/s.f., and the concrete, with what my guy called a "broom finish?", would be under 3 bucks s.f. This is a virtual wash. Then the cinder block would only have to be six high, and 65 around the circumference, which my construction dude said would be only 450 dollars delivered. Maybe I'm oversimplifying, but it seems to me like you could put a really cheap plastic around the outside of the cinder block, and back fill to it. This amount of dirt work should be any tougher (maybe even easier?), so where would the extra expense be? You could even put some conduit in through the base of the concrete to support a pond light, like what n8ly had, and an aeration line.
I need somebody to tear this idea apart and bring me back to reality. Remember that I want a pond that my water will cycle through to my Dad's pond that is about 30 feet X 10 feet, and about 4 feet deep.
