Yes, there is a charge to park; a little surprising for Midwesterners. However, I was told that the track itself isn't collecting the money- it's the fairgrounds who's taking that profit. One other thing that I wasn't prepared for was their pitpass policy. You can NOT upgrade your GA ticket to a pitpass for the extra $5 or $10. Not sure why (they couldn't or wouldn't explain), but I was told I had to pay the full amount and was turned away.
One other thing. The first time I traveled out to Perris, I stayed in Moreno Valley. Not bad (slightly rough), but after venturing around the area, we liked Riverside
way better. Been staying there near UC-Riverside ever since. A few more miles, but worth it IMO.
Finally, yes, there are traffic jams on the interstates 24/7. Have fun with that!
(Here's the link to my go-to hotel which is independent and will save you a few bucks.
http://www.dynastysuiteshotel.com/)