Monster Manual. Associated class levels are woth +1 CR per class level. Non associated class levels are worth +1/2 CR per clss level. Spellcaster class levels always count as associate if the base creature casts spells as a member of that class.I'll have to find which book talked about giving some monsters discounts in certain areas. I believe it said something about if the class being added on was way outside of the creatures specialty and the powers would not add anything to the CR then to give 2 levels for each point of CR, or something similar. It would make sense here too.
James McMurray said:There's no need to go any further. The rules explicitly state that sorcerer levels for a dragon are worth +1 CR per sorcerer level. House rules to the contrary are of course fine.