That's quite outrageous, I got my partner to cast my head at a total cost of about £30 :F
The hard part was getting the alginate mixed in the right ratio so it didn't drip off the face, the back of the head can just be done in modroc (plaster bandages) unless you're really particular about needing precise detail on the back of the head. 500g of alginate powder is more than enough to cast your face and costs about £10. Add to that a couple of rolls of plaster bandages, some plaster of paris, and a bucket of sand to support the mold while you fill it, £600 is a LOT of money for that, when the parts to make it come in at around 5% of the price!
I don't know if I can really help you find someone who can cast your head, but I can confirm that £600 is WAY too much to pay for it, unless they're casting it in bronze or something.