Theron writes:

I just purchased a 20x50ft piece of roofing rubber from a local DIY
box store called 'Menards' cost was $289.00 plus sales tax.

Just for fun I priced justliners.com for the same sized piece of
Firestone Pond Guard:

$400 plus $135 freight, Oklahoma to California. $20 less
if delivered to a business address. No sales tax, which
is a 8.25% bite in California.

(the freight charge is actually for a 30x30 liner which is 10%
lighter but I didn't want to wait for another email quote)

One thing about roofing rubber, I think it only comes in 20' widths so you have
to figure the time and slight expense to glue it if you want a wider piece.