Question: Can I use a cheaper liner than EPDM?
The biggest enemy for any liner is the sun. I have never found a liner that does not crack and break after a season or two except EPDM. Replacing a liner is harder than building the pond the first time.
I have seen ponds done with kiddie pools and yes, they work just fine untl they crack. The preformed ponds that you can buy at big box stores crack just as fast. If you must use a preformed pond, buy a horse drinking trough. Usually they are made by Rubbermaid and last forever. I had one buried in the ground when Katrina hit, left it there for a year. When I retrieved it, it was fine. I now have in my new front yard.