"If you assume that Apple sells 7.5 million units in the first year (midpoint of our 5-10 million unit range) at an ASP of $350 and a 30% gross margin, it would increase our 2014 revenue estimates by $2.6 billion or 1% and gross profit dollars by $790 million or 1%.
Morgan Stanley's Katy Huberty made a similar estimate last year but came up with a number 10x higher. She took 500 million iTunes account and estimated a 20% to 60% penetration rate for devices priced between $100 and $300 . This resulted in a sales estimate of 50 to 150 iWatches per year and $10 to $15 in additional revenue.
Huberty's calculations can be seen in the chart below...