Thinking of investing in a Litecoin mining rig?

I did a rough calculation and it seemed too good to be true. I would get a total return of $3500, a net profit of $987 while only investing $1700 (Almost 60% net profit!!!) accounting for electricity, depreciation, and increased difficulty (using data extrapolated for the previous 12 month) if I leave my machine on for a year.

This is more than the return gained from investing in the stock market. Shareholders are considered very lucky (or very skilled) if their shares increase by 50% per year.

So wouldn't this cause more people to come in causing it to become less and less profitable? If so, should I still purchase a Litecoin mining rig? What are your thoughts?