Is it cheaper to buy or build a loft bed?

Should I build or buy? I need a handyman or carpenter to advise me! I am looking to build a wood version of the following metal bed http://www.walmart.com/catalog/product.do?product_id=10928480 from Walmart. This bed is going to be for an iguana - she has her own room - who may wind up weighing 20+ pounds so the lumber need not be able to support a human.