I am a big mono mini fan and my favorite bag of all time is my mono mini navy Lucille GM. Keep your eyes open on ebay, reputable sellers do post mono mini navy bags (like the Josephine or Lucille) - just make sure to always authenticate!
I've also found that magic eraser works great on the minor dirt marks on mono mini. I have the same navy billfold, and magic eraser worked great on it. Make sure to test in an inconspicuous area first though, just to be sure!