In many standard backyard installations, switching to a reinforced liner like RPE allows you to skip the underlayment entirely. That's a big win for your budget! It works because RPE has a high-strength internal grid, giving it a puncture resistance that's significantly higher than unreinforced alternatives. In normal soil conditions (no sharp slate or excessive tree roots, for example) a protective secondary cushion is optional.
An advanced liner material like RPE lets you save on the underlayment itself, plus the time and effort you would have spent installing it. The combined savings often makes up for any price difference between the "expensive" reinforced liner and a bargain-bin option.
