Would it be an option to simply remove the tumor as best as possible without damaging the jaw, and wait to see if it comes back? If it comes back then maybe you could do the radical surgery then? $4-5000 is a lot of money too, and as much as I would be loathe to put a price on my own dog's life, I wouldn't be able to pay that big of a bill. With your dog already being a senior, maybe a less drastic approach as I suggested above may be a reasonable plan...
Roxy - Feb 2005 - 75 lb American Staffordshire Bull Terrier (adopted Jul 2011)
Peewee - Jan 2006 - 8 lb Chihuahua (adopted May 2009)
Myka - Nov 1998 to Jan 2010 - 85 lb American Pit Bull Terrier cross
Lacy - Sept 1992 to July 2003 - 18 lb Reg Shetland Sheepdog