I don't really think it's a matter of quality with the mark-up. Replacing a pool liner is labor instensive so that's most likely it. They have to account for removal, scrubbing down the sides with chlorine, cleaning lights, replacing fittings, repairing any floor issues, retaping side-seams, lots of guys to install, partial cost of pumps and shop-vacs, and a buffer for when things go wrong. There's probably not a huge profit margin.

Have your family guy get some friends to help and give a great BBQ afterwards!