There is small return on investment - no real profit

Formfill type libraries which could be set up quite easily with MSWord merges will yield very little return. Pro-forma precedent packages (for the most part) which are not tailored to your firm and require post assembly editing will probably yield a small return on investment - they are a "quick fix" to get you started and will take care of the statutory parts of your documents (court headers, required text and the like).  The key here is that its not your drafting and your expertise - you really are just producing "shell documents" that handle all the common stuff, rather than a system that is producing the content of your finest legal minds.

A document assembly system that reflects the decision making process of your professional staff will yield large returns if you have enough work to do to make it pay. You don't build a massive car manufacturing plant if you only wish to make 5 cars per year, you build them by hand - the same principle applies to document assembly. This return can be in the form of reducing staff overheads or increasing workflow to take advantage of increased capabilities from current staffing levels.

Larger profit margins come from leveraging your lawyer's intellect: embed their processes into a system that can be utilised by support staff. Reduction of proofreading times and less drafts to perfect a document means more profit margin and more importantly, more time for your professional staff to take on more work. Combined with "flat rate per transaction" type matters, or fixed scale matters, document assembly can yield impressive profits that will provide 100% ROI every few months.