If a necromancer mounts a corpse cart do you have to pay the 90 points to mount the cart and pay the price of the corpse cart? Equaling 180 points to mount a corpse cart.
No. The points listed in the Necromancer entry shows how much buying him a Corpse Cart mount costs (which is why it is 90 points, the cost of a Corpse Cart). If you pay for a character to have a mount the character gets the mount, there is no additional mounting fee.