Hi Aisling,
There are a number of things that will effect the cost of fish(all types) the same as everything else.
I personally would class the term Goldfish as a banner name.
you have comets, fantails and lionheads. here is a link which will give you more ideas what i mean
www.goldfishinfo.com/variet.htm
The other thing that must be consider is the time and thus the cost related to growing fish to this size. the cost of food and meds and any special food and of course quality of the fish in question.
Good quality fish from a respected breeder will always carry a higher price!
I hope this helps
Mickey