Looks like a car rental from Enterprise would cost you about $25 for one day (economy car). I just called, and they charge an additional $25 one way fee (probably just to cover returning the vehicle to the original office). That would get you to San Diego for $50 plus gas - which would probably be about 4 gallons, maybe 5. It looks like although most gas prices in SD right now are hovering around 3.25, but there are quite a few places with prices in the 2.9x range. Plan on another $20 for gas, and you've got a $70 price tag on the trip.
Sources:
enterprise.com
sandiegogasprices.com
As I recall, there's also a
AAA deal that provides transportation from
Disneyland to the SD Zoo... but you'd have to be a AAA member and stay on Disney property to get it.