Kind of funny. This ebay seller (who has 100% positive feedback) says the chip is rare although he says he knows nothing about it. $3 postage, too.

RARE CASINO DE ISTHMUS CITY $5.00 CASINO CHIP
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=63756&item=6176333874

He says:
"you are bidding on a $5.00 chip from casino de isthmus city.not sure what year this is from.or where.winning bidder must contact me within 3 days after auction ends.payment must be received within 10 days after auction ends.any questions email me."

Robert