You use them instead of coins for San Francisco parking meters. The $20 ones
are always $20 plus mailing fees. A company called Plum District: http://plumdistrict.com/deals/sfmta-50-off-parking-cards
is offering them today for $10 shipped (i'm sure to get your email address
and send you junk mail, but so what.)
I apologize - I posted this deal before with a personal link. This one is
clean. (reps always welcome, tho)