Hi Danmuss,
As Blayke said it is standard to charge for a written valuation. A valuer must test the stone, test the gold and write up the certificate. You are paying for their time and expertise.
Amethyst is a readily available gemstone with the paler ones being cheaper than the deep rich dark purple. If you dont want to pay for a valuation you can try and find something similar for sale online or in a shop which would give you an idea but keep in mind the second hand market for jewellery is approx 1/3 of retail price.
Have you got a photo of the piece you could post?