BMW M3/4, how far below invoice?

I was just wondering if anyone, especially the brokers knew how far below invoice they have seen an M3/4 sold for before rebates.

I have seen the other forums discussions on pricing but they are generally happy with $500-1000 above invoice. I have also seen some sold below so I know it is possible.

Having just leased a BMW X3 at $2000 below invoice I was intrigued to find out others experiences with M cars.