One of the most popular pages on this blog is the one on filament volume and length. But, I didn't break down the cost of plastic per volume in that post. Since several people have asked, the really short answer is "about a dollar per cubic inch".
These are ball-park numbers, but the quick take-away is that it's about a nickel per cubic-centimeter, or 75-cents per cubic-inch:
A 1 kg spool of ABS is ~960 cm^3, while PLA is 800 cm^3.
At roughly $40/kg, ABS is roughly $0.04/cm^3, and PLA is $0.05/cm^3. For non-metric folks, that puts ABS at $0.65/in^3 and PLA at $0.82/in^3.