Need some very basic advise on cost calculations for equipment purchace

Trying to justify the purchace of a conformal coating machine for several of our PCB assemblies. We have been performing the operation manually at a cost of  $10 per board less than that quoted by our board builder. My estimate is that by automating we can reduce the price per board another $2 below the quote. So for a $50000 machine, we can recover our investment by producing (50000/12) = 4167 boards. This gives a payback in <18months. Sounds great! also sounds too easy. Am I missing something?

Thanks in advance!!!!!!!!!
Bob

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