It’s been that only vehicles of >12t subject to electronic toll collection so far; from 1st Jan 2010 the it’ll be everything above 3501kg.
So if you go that way with a van or a 7.5tonner don’t forget to stop and get your box.
premid.cz/
As of the rates, the government wants to differentiate the rates for days and times - Friday afternoons will be more expensive than any other day & time; nights should be cheaper than days etc.
At the moment it is not clear whether this charging system will come into effect on the 1.1.2010 or not.