Maybe the high volume people might have an arrangement, don't know.
I have to carve it out of my Adsense check each month and it ain't easy. I used to spend $500 per month, now I spend $500 per week and they suck it right out baby. Just resigned myself to banking the 2K each month and I am sure it will progress from there.
You may have to spend a bit more with Adwords before they will allow monthly billing - not quite sure what the amount is thou. My monthly spend is in the region of $18K if that is any help.