Usually Lexus sells the complete set of maps for Europe in 4 discs, north, east center and south: and every disk have some countries that are present in other disc too, so it often happens that, being the whole set expensive, someone sells on ebay a single original disk that is for him useless; as the disk reader of GS reads only original disk the only cheaper solution is to use a original release.
If you don't want last version, it's easy to find at a more reasonable price a dvd of a previous version, like 2012-2013 or similar.
I agree that maps from Lexus are too expensive 8-)