I think it's a rip off! TomTom should have updates for 10USD max! and they know it! approx the cost of a new updated paper map!
TomTom should definitely do a better job of letting users know exactly what changes have been made, so that their customers can make informed decisions regarding whether a particular update is worthwhile. Garmin, for example, allows you to view the maps online. While this is better than TomTom does, it still provides no information with regard to non-visual data, such as turn restrictions, speed limits, etc.
However, I think it's unreasonable to expect map updates for $10. First TomTom must buy the maps from their map provider, and then manipulate the data for use on their systems. The current price of $100 is far too high, but $10 is unreasonable low.