Dealer is wrong.
All you need to do is get the right key blank, get it cut and transfer the transponder.
Total cost probably under £15.
I really get cheesed off with dealerso knowingly(more than probable) trying to fleece folks.
https://www.youtube.com/watch?v=Yk21gcxxcCs
Don't forget to glue the cover after transfering(as mentioned in vid) transponder.