This article is from 2011 and provides great pointers concerning what should be included in your budget justification. The article can be found at: http://researchadministrationnation.com/2011/12/30/justification-nation-how-to-write-a-strong-budget-justification/

*Please note, 2 CFR 200 will go into effect on December 26, 2014. This regulation actually relaxes the restriction on computer purchases, which is used as an example in this article. However, your justification should still tie any expense to the actual project being performed.