I think a better way of working it out would be to use an average daily profit to work out what would be more profitable rather than an arbitrary number of coins that way it could be used for multiple coins, so in this scenario for the mining option it would be $15 per day @ same cost. So anything over $11.50 per day profit better to mine than buy, and you will also have a mining rig left over at the end of the year to continue mining with. But as you said with the other variables to factor in its near impossible to predict coin prices or difficulty increases.