Regiatration is just a paperwork thing and the costs are very low .
The problems start with getting the C of A you MUST ,MUST & MUST get an export C of A from the USA , the aircraft must meet the type data sheet spec or you are going to be into a lot of delays sorting out the mod state.
The local CAA office will send an inspector to check the aircraft out , if all is well then the fees are the ones relative to MTOW that you will find in the airworthiness notices.
Dont let this put you off importing an aircraft but you will save yourself a lot of trouble if you buy a compleatly standard aircraft , all these go faster mods that you see will just cost you time and money getting them past the CAA.