I think with the right customer it works ok, but in general it can lead to discussion about how cheap it is, and soon becomes more about saving money than about getting quality signs and service. Before you know it, you've both spent half an hour talking about signs and $$$ and not making them. Sorry, I shouldn't let my bad experiences affect your idea, but just thought I had to say what can happen quite easily.
The only way I would offer this type of thing, if they were done off templates, the customer saw an exact sample of the product in your shop which they would touch and see and approve. ANY deviation from said template and/or materials/methods - double the price... heck, triple it.
This can only make you money if it's heavily standardised to the finest detail.
My 2c