Complexity Index of PCB for characterising reflow profiles| 11 August, 2003
Our old friend Bob Willis posted at his site [ http://www.bobwillis.co.uk ] a document that may be a good starting point. In the "Download" section, look for "New Product Design Evaluation Form (Word.doc file)".
Certainly the number of solder connections per square inch is a measure of complexity, but that may not get at what you're seeking. Possibly pre-reflow board weight per square inch, with a factor for board thickness?