I bet he's going to be washing his Donkeyhorse in it.

The old style copper bathtubs (from before indoor plumbing) were often made of copper, and weigh on the order of 200 lbs (without the 65 or so gallons of water they hold). So, whatever 200 lbs of copper cost, plus some for crafting it into a bathtub.
Here's kinda of a measuring stick:
50 coins to a pound (doesn't matter, gold, silver, copper).
So 200 lbs of Copper would be 10000 cp = 100 gold pieces.
Well, that depends on the design of your coins.
50 coins per pound, then, are some pretty large coins - the weight of modern half-dollars or so.
SRD 3.5 :: Equipment said:The standard coin weighs about a third of an ounce (fifty to the pound).
But of course, D&D 3.5 already states the cost for a lb. of copper.
Call me a simulationist, I guess. Coins of that size are still just friggin' huge, to my mind.