Are there any cheap or free parking at walking distance to the convention center?

Nope. The best thing to do is to take the lynx blue line to the nearest stop. And then walk to the convention center.

There's a parking deck right across the street and it's reasonably priced.

Train is best

Train is best bet. Parking goes from 8 to 20 a day depending on lot. Weekends you can find it as low as 6 within a block

Train would be preferred, but parking is near...