I have a client who wants me to make the Romantic Castle cake. It is a Wilton kit. I have absolutely no idea how much to charge for it.
I charge $20.00 extra to decorate the towers (fondant on the tops and outline windows, etc. with royal icing) on top of my normal tiered cake prices.
FYI those things won't stand up in the cake by themselves, you need to drill a hole in the bottoms and hot glue a dowel rod in there and then use the dowel to get them to stand up in the cake, a pain but once it's done you dont have to worry about it again.
Get a deposit for the kit, I let mine go out without a deposit and never got it back or the hour I spent drilling holes and glueing dowels, etc.