Scott,
You fail to mention trace widths. The thinner the trace, especially
on the outer layers of the board, the more you pay also. (due to yields)
Can you get the remaining 4% routed on the outers?
Is this board just a prototype, or mid-range volume, or large qty production?
These are also factors in the decision.

I was given some basic cost ratios which I'll pass along.
	All std mat'l FR4 180tg
	DSR	1.0
	4 lay	1.7
	6 lay	2.6
	8 lay	3.5
	10 lay	4.5
	12 lay	5.3

I haven't tested these ratios but have used them for estimating.
If this board is large qty prod. or well-defined (meaning it's been through a
revision or is basically known circuitry) then the decision to go to 4 layers
might be a consideration - but the 6 layer would be a better choice from the
quality standpoint - Here's where we all talk about "the cost of quality"...

Tom Kavendek
Lucent Technologies
Bell Labs