I've found that most of the Rube Goldberg's I see have only opted for these homemade contraptions because they feel they can do something as good as (or better) than the manufacturer much cheaper. In some cases this holds true, but I would say that those instances are the exception rather than the norm.

Yes, there are plenty of examples where it would appear that the manufacturer is overpriced. Still, who put in the man-hours for design and construction? Only when economies of scale come in does the return on investment become positive and prices drop... but I'm not an economic's major, either!