It's a very tricky question. When I applied a few years ago, there was an entire 4500 character essay devoted to this. Now, I believe you only have 600 characters to discuss this issue.
Few things come to mind :
a) Was your family making under 150% of the federal poverty level? ( I think the poverty level is somewhere between 35 to 40K)
b) Did you face difficulty adjusting in the wealthy school?
c) how did worrying about money manifest in your lives? (e.g. did you have to forego meals or had severe money issues at any point in your lives?)
If you answer yes to any one of these, you should write a disadvantage essay. Don't be shy about writing it; but also discuss the fact that you are a strong candidate DESPITE your disadvantaged situation. Don't whine about being disadvantaged. Show what you have achieved despite all the challenges you and your family faced.