I can tell you this is not the case from first hand experience. Even removing all the custom pieces from the polisher I made, you can see the materials to replicate a Diamond polisher are over $200 without even considering the wood to construct it. Add in the wood and you are approaching $400, if not over. Add in the custom bits I added like the stainless trim, laminate, feet and a few other details and you are in the $600 range in. I already had the wood so that wasn't a factor for me. You also have to factor in tool availability. Not everyone has access to the tools required to construct one.
IMO, if you are not interested in aesthetics and can fade the delivery time, you're better off buying a Diamond polisher vs. building your own. Despite the perceived high price tag, it's a good value.
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