The cost of EPDM depends on the specific treatments applied and the intended use. EPDM pond liners can be more expensive than other options, but will also last for decades, so you may spend more at the outset compared to cheaper options, but over time it will prove to be an excellent investment.
Cheaper pond liners are less resistant to tears, UV, and ozone exposure. They're less flexible, which makes them difficult to conform to tight corners, unique borders, and submerged shelves. EPDM is also simple to repair if that becomes necessary.