Well I personally took the liberty of sending an addendum about this to explain my grades. You read mine and it was under 1 page.
What I would recommend is not getting too detailed, and definatey not repeating things that are on your apps (i.e. you went to 5 schools, no need to list all of them on there etc.). I think the main theme of the essays seem to be to get them as short and concise as possible to keep the addcoms from spending too much time reading them.
As far as numbers go, if you feel that it helps your case a lot, I may use it. Personally, I just explained that I had trouble paying for everything and even almost had to drop out of school, hopefully they'll figure that the amount of money I made compared to my costs were pretty bad.
Of course though, I'm no pre-law advisor, and I've actually never spoken to mine because she is hard to get ahold of (our school sucks).