Just grabbed my office's Henry Schein catalog to get you some 2009 pricing numbers here. For arguements sake, we'll asume that you'll use the same basic supplies (2x2 cotton squares, cotton rolls, anesthesia, etc) for each.
The Tytin Sperical Alloy, double spill capsules(what I use in my office when I use amalgam and argueably the "standard" amalgam on the market) has a cost of $119.99 for 50 capsules - lets say that 1 capsule will restore 1 tooth(although with many "standard sized" preps, a double spill capsule will often get you 2 or 3 teeth restored) - so the material cost is about $2.25 per tooth.
For a composite, I'm going to for cost sake, I'm going to use the pricing for Kerr's Premise Composite (a universal nano composite), which is what I use as my primary composite material. A unidose refill kit is $99.99 per 20 unidose ampules and once again for arguements sake, we'll asume that 1 ampule = 1 restored tooth. So even before you factor in cost of bonding agent and etch, you're upto $5 a tooth for materials. Figure $7 to $7.50 per restoration by the time your done materials wise then. Also, depending on what brand/type of composite + bonding agent you choose to use, you're costs could be a few $$ less per tooth OR a few $$ more per tooth.
The other factor, is that because of the increased steps involved in placing a composite, you're using more chairtime to complete the same restoration, and that IS a factor that needs to be considered too since for myself as I'm planning out my schedule I know that on average due to the increased number of steps and curing time, I generally plan that a composite restoration will take me an additional 5 minutes to complete compared to that exact same amalgam restoration.