There are many opinions, and books, about that.
See
http://www.google.com/search?hl=en&q=information+technology+budgeting&btnG=Search&aq=f&aqi=&aql=&oq=
How may financial managers budget for unforeseen changes and improvements in information technology that require large capital outlays?
2 answers
How may financial managers budget for unforeseen changes and improvements in information technology that require large capital outlays?