All of the fares of my mileage runs are in USD, so I calculate the cents/mile ratio in US dollar.
My personal threshold is less than 1 cent per earned mile if status miles are not my objective. For status miles, it's under 3 cents per mile.
It's my opinion but I find that it is really not hard at all to price runs that are 2 cents per earned mile. Elite bonuses (100%) help and special promotions can help but sometimes the routings are just too darn good. For example, I booked me a $151 NW SEA-BNA fare that would net less than 2 cents per status mile (hence less than 1 cent per earned mile).
Oh, and I do take into account discount certificates, vouchers used when calculating the cost. However, I do not take into account the bus fare from Vancouver to SEA, any layover costs (usually no hotel for me), and other travel costs (meals).