I am sure it is by weight. Typically, plastic spools go for around $18-35/kg. Every company seems to shy away from guaranteeing length. But, if you know the volume of the part you want to build, it is almost a 1:1 transfer of material.
Lulzbot says, "The LulzBot AO-101 is one of the few desktop 3D printers on the market that can print with ABS and PLA plastics. With some simple adjustments, you can print with a wide array of experimental materials like wood filament, nylon, polycarbonate, polystyrene, and glow-in-the-dark materials. "
I am sure they will have to verify the type of materials first. So, I don't think this will be printing stainless steel from spools any time soon.
The 3D printing revolution seems to have a knack for quickly moving technology ahead by way of collaborative effort and even a little friendly competition -- all of course in the name of scientific advancement.
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is