What do you expect to pay a local shop to build you a system?
Will they use the proper coated pipes that won't rust ? How much will the pipe cost. Do they re-coat it after welding ?
Will they have the knowledge to build a well flowing system from years of experience ?
Will the have the correct heavy duty hangers to correctly support the system ?
Do you plan to use Cuda through-the-valance tips? How much will they cost ? Will they be Stainless ?
Ask yourself these questions before you disregard the idea of buying one of those "expensive" systems.