I price it the exact same way as I do resi work, but wouldn't take it on if he only wanted it done every 6 months. If you do take it on, consider charging him double, as it will be like a first clean every time.
I don't charge double on first cleans as I look at it like an investment in them, and its only the first time that it is a bit of a hassle with mucky jobs. But for every booking to be like that I would have to charge double. Again though I personally wouldn't take it on.