That page gives an overview over the (dead tree) literature on the topic:
<a href="http://forums.construx.com/blogs/stevemcc/archive/2008/03/27/productivity-variations-among-software-developers-and-teams-the-origin-of-quot-10x-quot.aspx" rel="nofollow">http://forums.construx.com/blogs/stevemcc/archive/2008/03/27...</a><p>An interesting data point he cites is the comparision between Lotus 123 version 3 and Excel 3.0: "Excel took 50 staff years to produce 649,000 lines of code. Lotus 123 took 260 staff years to produce 400,000 lines of code. [...] The difference in productivity between the two teams was more than a factor of 8." 
But then it will probably not be too difficult to find people who will tell you that 123 was still the better product.<p>[Edit: Another important point is that the these factors are between the best and the worst. The difference between the best and the median can be considerably smaller, so it is not that surprising not to see any programmers paid ten times the average.]