Various materials are used by different manufacturers. There's no need to worry too much about this though - just find out the length of guarantee, which is a good indicator of durability and resistance to ultraviolet (UV) light (the part of daylight that causes plastics to deteriorate).

Pale coloured liners are more susceptible to UV degradation, so choose black in preference to shades of blue or stone.

Cost depends on materials, but also shapes and sizes. In any case, there may not be that much choice if you're looking for a specific shape and depth.