According to the 3.5 PHB, for multiclass characters the skill point cost to raise a skill is determined by the class you are leveling up and max ranks are character level + 3 if a skill is a class skill for any of your classes, and (CL + 3)/2 otherwise.
Example:
A level 1 Rogue/3 Wizard has gained enough xp to level to character level 5. He takes a level in Wizard. He has 7 ranks in Disable Device, and spends two skill points to raise disable device to 8.
Andy Woo: A. Max ranks are determined by the best of any of your classes, as it says in the PHB p. 62