Speaking as a fan that hangs around the edges of the modding community (and as someone who did a tiny bit of voice acting for REDFlame), I cannot answer the question about hiring that particular voice actor, but I do know that if you talk to someone from the REDFlame Interactive modding team, they could probably give you good advice on prices, and on obtaining professional voice actors (and maybe even one that sounds generally like the one who did Geralt's english voice). PaulV would be one to talk to.
It doesn't give his rate, but you can find out more about that actor here: http://www.imdb.com/name/nm0168339/
thanks for you two, ok first he live in california, so for me (France), that mean paying actor plus studio in USA... probably too much. it's just a question, if modding community continue to grow, could it be possible to pass through CDprojekt to package our dialogue line, and make a whole record session? I think it probably too much work to handle hit.... but hey it's cristmas soon, everybody can dream... 