Hi Suzanne, thank you for your message and for your interest in our light kit for the LEGO Disney Castle. You can find the product options here:
The final cost depends on the options you pick. You can purchase the exterior kit only, the interior + exterior kit, and also choose whether to add on fireworks or not. You can see the various options on the product page above.
Don't forget to add a power supply. We recommend the USB power cable.
Please let us know if you have any additional questions. Thank you again!