What are the costs per meter of filament for PLA, ABS and PET?
This strongly depends on the cost of the spool in question. Prices are not consistent among materials at all; a spool of ABS can cost anywhere from \$15 to \$60 and the same price range applies to pretty much all other plastic.

For the purpose of this question I am going to assume that a 1 kilogram spool costs \$25 (regardless of material) but you can scale the figures to what is appropriate for your brand of filament.

ToyBuilder Labs lists the density of ABS at 1.04g/ml, whereas PLA is somewhat more dense at 1.25g/ml. As such, a 1kg spool of ABS would be 400 meters (1.75mm filament) or 156 meters (2.85mm filament). PLA would come out to 333 meters (125 meters for 2.85mm filament)

We thus have the following costs (cost per meter for 2.85mm filament in parenthesis):


ABS: 6.25 cents/meter (16)
PLA: 7.5 cents/meter (20)


The density of PETG is roughly the same as PLA, so they have the same cost per meter (for similarly priced spools).

In general, to compute the cost per meter, you would use the following formula:

$$[\text{\$/m}]=[\text{cost of 1 }kg] \times [\text{density in }g/mm^3] \times \frac{[\text{diameter in }mm]^2\ \times \pi }{4000}$$