I have periodontitis and I've had deep cleanings (or cleanings beyond the regular, I don't recall exactly what they were called), and I don't remember them being much more than $100, $200 max. $800 sounds VERY high. Have you shopped around? I'm surprised nobody on this forum has commented how high that figure seems.
I've cobbled together a treatment for my perio from self-help books and my periodontists instructions that seems to be helping me a lot. You may want to take a look at http://www.groups.yahoo.com/group/periodontitis,
My Periodontitis Diary.
Do call a few other places. Maybe 1-800-DENTIST and tell them your financial constraints. Good luck. James