@L0akum Realize that even with a car rental you gotta pay for parking,which is 10 to 15 dollars a day to just park it by con. Odds are you will have to park at a satelitte lot and take the shuttle in. I'd save the money you spend on a rental car, take a taxi to downtown, and use the free shuttle provided by con to move around from there. Plus staying farther out, like in National City or in Hotel Circle, the cab won't be more than $20 one way. So you figure cab ther and back per day is $40. You can save that a day by getting the cheaper hotels that they offer. Most of the Con close stuff is $200 or more a night, stay out side and you can find stuff at $130 a night. That plus parking adds up.
To be honest, unless you want to do other stuff, like hit the beach or whatever a lot, everything you need is a quick Comic Con shuttle ride or walk into the Gaslamp. Leverage what you need. I park but that's cause we drive up, but we don't touch the car from 8am to when we leave, usually after 11pm...
Hope that helps. Under the OMFG meetup section I wrote some of my best best for comic con, you can find it here
http://www.oliviamunn.com/forum/topic/comic-con-2010
Its a big thread, but good advice throughout