FWIW, in the area where I live (central California), we have stores called "Falling Prices". These are discount stores that buy overstocked pallets from Amazon, Target, etc, and offer items to the public for prices ranging from $6 to $1. I have picked up 2.2kg rolls from there in the past many times. The filaments are often PLA, but lately I've seen lots of TPU too. In fact the past 2 weeks I grabbed up 5x 2x2kg rolls of amazon basics tpu, priline flex, and even a roll of ninjatek cheetaflex for $4 to $6 each (also Taulman Bridge, various ABS brands... hoping to find 3dXtech stuff someday lol). I have a big rubbermaid storage packed with assorted filaments since I found the place. If you have one nearby, you should check it out. It's kind of a "dumpster diving" experience, filaments are not guaranteed to be found, and you sometimes have to dig through items to find them. That said, most of their clients are the type that toss filament and other 'tech valuables' to the side, at least in my area.

On a side note, Falling Prices also sometimes has other items of interest to me... like I've found several esp32 boards, arduinos, servos, an SKR mini w/ touchscreen, etc... no BNF whoops yet though lol. The items can sometimes be high value too... I got a $400 level-5 brand drywall finishing box (I do construction for a living)... a makita lithium powered finish nail gun (works great)... full boxes of collated framing nails... lots of industrial LED fixtures, etc. My wife often comes with me and finds designer items, like $100 pairs of shorts for $6.

Anyhow, I know most folks won't have a Falling Prices nearby, but maybe there are other similar business models operating in your area? Central Cali does have a lot of warehouse/distribution activity to feed such a business... chances are if you live in a similar shipping hub, there may be something similar. I'm curious to hear from others who may have found similar cheap sources of filament.