I agree with Kronikabuse - you do save some on lower end stuff, but the real benefit to Filastruder is saving on the more exotic materials and/or using better quality raw materials. Eg - that $9-15 per KG PLA filament you mention is almost certainly NOT using Natureworks 3D850 or 3D870, yet for $11 you can buy 1KG of 3D870 pellets. Or another example - say you suddenly decide you want to see what happens when you make wood filled PETG (instead of the traditional wood filled PLA) - you can (just make sure you don't have the filter in on Filastruder and use a larger nozzle on printer). Wonder what happens when you mix in two different colors? You can.
Recently I saw some filament called 'marble' filament - it's mostly white with black specks. I threw some extra fine glitter in and made sure that it would print fine still. Now I just need to get some black colorant, throw that white glitter in, and see what the opposite looks like (I'm hoping less marble, and more granite).