E90 M3: $54,575 MSRP
E70 X5 4.8i: $54,500 MSRP
E70 X5 4.8i: $50,140 Invoice (Edmunds)
E90 M3: ~$50,200 Invoice (est.)
E92 M3: $57,275 MSRP
E70 X5 4.8i + ZPP: $57,150 MSRP
E70 X5 4.8i + ZPP: $52,550 Invoice (Edmunds)
E92 M3: ~$52,600 Invoice (est.)
Just estimates, don't sue me if they are wrong.
You could do this a million other ways too, especially if you average the percentage difference between MSRP and Invoice over all BMW models. Or maybe you could just do M models too (not sure if it'd matter). But I figure this is probably almost as good and I am lazy.
Now as for why you'd need this? Well you might not, at least not for a while. But there will come a time when its possible to buy an M3 under MSRP, so this might be handy then. Course, we'll have real invoice by then, but hey, I was bored.