I don't think the cost of the micro controller is too much of an issue here - I would think your labour cost would predominate. If you were going to make 100's of these things maybe the cost of the actual micro may be an issue. In 'one-off' jobs I look for the easiest ,most straight forward solution that saves me time - if I need 10 micros at $100 ea that will save me a week of time, then I'm ahead. Engineering is a balancing act - it's a matter of applying the right balance for the task at hand. Ultimately it all comes down to cost: be it cost of the project or the cost of a manufactured unit.