Coodgee, if you can wait to find a 2nd hand one you'll save heaps. For a new 40L SS urn, expect to pay $250 upwards.
You could also consider whether you want to heat 40L in one batch or whether a smaller unit will do. For example, if you're not going to use in excess of 20L for either the mash or the sparge, then a 20L urn will do nicely - ie heat the sparge water while mashing. I recently picked up a 20L SS urn in excellent condition for $40.
Other places you can look for a 2nd hand one include: catering suppliers (most deal in 2nd hand), catering equip auctions, farm clearing sales and second hand shops.
Some alternatives to an urn might be small domestic/marine water heaters (eg office, boat, caravan types), wall mounted urns (esp at building demolitions), pharmaceutical heating units etc.