I don't know that the full numbers are well known, but this Ars article reveals some of the details:

http://arstechnica.com/microsoft/news/2010/05/apples-stock-r...

FTA: All told, Microsoft spent a little over $151 million to acquire 18.2 million shares of Apple stock, for roughly $8.31 per share. Microsoft confirmed that it sold all of its AAPL holdings some time ago, and likely did so at a healthy profit—after all, AAPL has traded significantly higher than $8 for many years. But what if Microsoft had held on to that investment just a little longer?

They would have made a little under 1.2 [edit: 9.7, typod math] billion dollars if they still held the investment today. Which would increase their cash and short term investment reserves by about 2% [edit: 10%].

Microsoft is printing money. Cash is not what they need or care about.

[Edit: math was corrected above] I'm not following your math, 18.2 million shares of Apple today would be worth about $9.8 billion--for a $151 million investment. I'm sure there are nuances that I don't fully grasp; but that's a huge discrepancy.

FWIW, I didn't mean to imply that MS should have held onto that investment; I was only answering the question about the purely financial ROI.

On the other hand, the ROI on investing into Apple might compare favorably to a lot of their actual business moves since 1997.

The appellate overturning of the DOJ's successful case to split Microsoft up was surely worth more than any investment in AAPL. These aren't directly tied, obviously, but a seriously infirm Apple (at the time, the only serious competitor in the consumer computing market -- this was before Linux broke out and long before smartphones) might very well have changed a few minds in the 9th circuit.

Completely agreed, even if all they did was break even on the deal financially (or honestly, even if all that money had eventually gone up in smoke) the value of having a real competitor to point to at that time was worth every penny.