For an Elite 15 a relatively cheap plastic bag full of polyfloss from the LFS may be enough to last a very long time, so this next bit might be overkill, but another bit of lore about polyfloss is that you can get it really cheaply in the sewing/housewares sections sometimes of big-box stores (like Walmart in USA) where its sold for stuffing pillows. I'm not too sure about this but I think the trick is to somehow figure out that it hasn't got fire-retardant or a few other types of added chemical coatings but that its just "plain," whatever that really means.
And yes, CharlieP, polyfloss should be considered to be just "different" from mesh/foam/sponge. Each individual type of media will be better or worse or non-functioning at carrying out each of the three major functions of a filter (mechanical filtration, chemical filtration and biological filtration.) In fact, we tend to think of their being one or two "best" types of media for each function, even though even those types of media will overlap with other functions they can do.
So for instance, sponge competes with ceramics as one of the top choices as a "biomedia" (because its a great surface for tons of biofilm/bacteria) but it also has a significantly good ability as a "mechanical" media because plenty of debris can get stuck in it. Differently, polyfloss is considered to be a top choice for "finely-sized particle trapping" or "polishing," so its a star in the mechanical filtration department. Polyfloss is not a great biomedia because it gets so filled with debris that it bogs down and must be thrown out and anytime you do that with biomedia you lose your bacteria. Polyfloss will have plenty of bacteria in it but it shouldn't be "counted" as a significant source of your biofiltration because you will periodically throw it out, unlike sponge or ceramic. So polyfloss is better used in a thinner last layer sort of situation in conjunction with sponges etc.
~~waterdrop~~