First off, your math is off (by a factor of 2). Secondly, there are probably charging Obama the amount of commercial money they would lose during a 30 minute spot. Commercials are what percentage of a 30 minute spot?
I hope he is paying a pretty penny - I seem to remember this coming up before: Are the networks obligated to charge him their usual ad rates, i.e., not make him a sweetheart deal to the detriment of the McCain campaign? Otherwise, donating cheap time should be considered a contribution.