I don't think so Scottish. I vividly remember an interview with Warren Spector in which he stated it sold barely a hundred thousand.
Then you vividly remember wrong.
Here are North American Sales figures for the year of 2001 only: 91,013.
http://www.agdinteractive.com/pub-03.phpGiven that most games sell most of their copies during the first six months of release, we can guess that several hundred thousand copies were sold in the US in 2000. Factor in world wide sales, online sales, and sales of repackaged versions of Deus Ex, 500,000 units sold is very conservative; 500,000 units sold is pretty damn good for a PC game. With 1999-2000 level development costs, Deus Ex would've made a sizable profit for Ion and Eidos.