I used to do a lot of work for law firms, and they were all cheap and paid slowly. (So why did I keep working for them? Some I didn't, and some weren't as bad as the others.) Present it as easily digestible numbers -- 16 x $xx per headshot, and 3 hours of office shooting at $xx per hour, for PR usage, etc. That's the way I always had success with law firms, or other non-art-industry clients for that matter. The actual numbers weren't too important, just so it wasn't outrageous, they just want to see exactly what they're getting per dollar spent.
I have done some law and financial firms and my rate is $125/head plus $250 trip charge. The trip charge keeps them from calling you in for one or two.
Also, detail how they will receive images. For example, I typically do a B&W and color of one image for each. If they want it resized for web, add a fee per head.
Detail any post work x per hour billed in half hour increments etc.
My contract say I am paid the day I shoot. Never deliver images w/o being paid.
Law firm are notoriously slow pay.
