I have registered two GS's in France and my experience was completely different for both - and different from the one described above, so it would be best to check with your local Prefecture exactly what is required for your department/region. My experience was with the Haute Garonne (31). The first bike I registered was an '05 1200GS in 2007. The Sous Prefecture accepted the Certificate of Conformity that I had got (free of charge) from Vines and the process was straightforward. Total cost €165 for the Carte Grise.
The second bike was a 2002 1150GS, which I bought in the UK in December 2010. The process was less straightforward because the homologation number on the UK CoC was not recognised on the French system. I had to write to BMW in Paris, sending them a copy of the V5 and a photo of the VIN number. They then told me what needed to be modified in order to bring the bike up to French norms. Once I provided proof that the work had been carried out, they sent the CoC. Again, there was no charge for the Certificate of Conformity.
In the Haute Garonne, inspections are done by the DRIRE, not by a dealer. Roynie had all sorts of fun and games registering his 2008 Triumph Tiger 1050cc!