Registration is with the traffic dept. who have a fixed rate.
While Insurance is with an Insurance firm, all will charge the same depending on make, model and year and is regulated by the Central Bank. After that is the value of the vehicle, what you pay is one thing and what they recommend is something else. Better to check with each and make your mind accordingly.
Also be mindful, if the vehicle has been in a "Third Party" and you want Comprehensive, they may charge a premium or deny.
Depending on how long you will reside in Qatar, and your travel desires, (Picnics, beach, dunes etc etc) you may want to consider a brand new showroom vehicle. Just my thoughts.