I went today to one of the prometric test sites in my country to register for the SCWCD exam. The guy there asked me to pay $25 as a registration fee. I do not recall payig anything for the first exam (SCJP) other than the $150 for the voucher, which I took at aother test center.
Voucher costs $150 (may depend on promotions), after you purchased voucher, you're regigistering online in the nearest Prometric testing center (http://2test.com), testing is free - you don't pay anything to Prometric center. This is how it normally works.
I know that in some countries, Prometric testing center may ask you to pay some extra money. This is country-/testing center-specific.