McConnell suggests in "Rapid Development: Taming Wild Software Schedules" that on large projecs the actual cost of coding and unit testing is 15% of the time spent.
So for a software estimation a good rule of thumb is
Jack Reeves takes another perspective on this he says that source code is actually a design document and that the compiling and unit testing is the building (construction). So we can conclude that Construction of Software is actually very cheap and in software the design is the most costly activity in contrast with manufacturing and construction (engineering) where construction is the most expensive activity.
Source: Martin Fowler: http://martinfowler.com/articles/newMethodology.html