Hi, HM has hit it on the nose as far as sources go. I can tell you that Grulla is a very fine maker, highly regarded in the spanish gun trade. As to pricem hard to say without seeing it. Condition and features will influence price to a large degree. As you may or may not know all Grullas are sidelocks and do come in various grades and features. If the gun is in mint or near mint shape and being a 16ga the price is a good one.
You can also try over at
http://www.shotgunworld.com and look under the I love my Spanish Guns sub forum - good info there.
You may also want to go to some of the gun selling sites (Gunbroker, Gunsamerica etc) and check out and compare prices to the gun your looking at and go from there.
All the best and Good Luck!
Greg