Of the dozens of large speaker cabinets I have built in my life, without exception it is orders of magnitude cheaper to build rather than buy factory speaker's retail. In every case, my constructions are much stouter with unique design features and functionality compared to the factory cabs. That includes using factory drivers and parts. But as donprice states, it takes your time, tools, and expertise. The only way it would cost more than 50% of retail is if you massively screw up and need to scrap and re-work, and perhaps if the cost of the tools is factored in. But as in any production build, the more you build the more the unit cost, and time, goes down. Also, if the speakers are mass produced, small and inexpensive (think cheap bookshelf size or smaller), then it is hard to match retail costs with DIY builds.
I would buy (and have) factory speakers when the construction/design is very unique, and I don't have the time, tools and skills to re-produce them.