There are labs and tutorials that often have "participation marks", which means you get marked on whether or not you go. They can be up to 20% (maybe more). As for most lectures, no one knows or cares if you attend. It's fine to miss lectures every now and then, but you generally need to go to them.
As Sara said, it works out to be $20-$30 per class. If you want to avoid skipping, just think of all the money you'd be wasting (if you would just otherwise be sitting around, doing nothing)