You can't get an average of the cost. It completely depends on what route of construction you take, how much detail you put into it, and so many other variables.
Just to be safe I'll go ahead and say at least a few hundred dollars just on supplies, and not counting time spent, for the average costume made from pep. If you have plans to mold it, go into the thousands.
inb4 "no i maed my armer fur 25 dullrz"