The top response is interesting that you can't make claims based on specific hardware without knowing the specs. Point taken.
I used 10% to show what 10% would do for the example, I would be surprised if many mainstream ones are 10% lower. But we're talking about a market where people jump on a combination of super cheap and super light.
12oz, $20, same power specs as two NB10000. The NB weights 85% as much for two but costs $100
Light and cheap comes with less robust batteries, less robust circuitry. Do you really think they didn't cheap out on the boost conversion circuitry to save money if it's that light? Something had to give in the equation and it's probably everything inside it.
For the sake of your point, would you rather spend $100 for the same thing or try an adaptor and see if it gives you more battery life and you don't have to stop as often?
And say you bought a battery bank from that brand, that looks more like this (I know it's a better brand, just look at the port setup)
Guess what you just gained with the adaptor, the ability to carry all USB C cables for input AND output.
Doing more with more and now your input and output match
You might consider attaching a portable solar panel charger kit to your bag and using it to charge a USB battery pack in your basket:
Any maker of these panels:
https://www.amazon.com/BigBlue-Charger-Foldable-Waterproof-Compatible/dp/B07ZQXPP17/
Connected to any maker of power bank batteries:
https://www.amazon.com/Anker-Ultra-Compact-High-Speed-VoltageBoost-Technology/dp/B07QXV6N1B/
That thing has a 5V/1A output, which may or may not work. Raspberry Pi's like an output that can supply more current (they recommend a 5V/2.5A power source o believe), but I'm not sure how they know the max current output of the power bank. It's safe to use that battery just for testing, but it might not be enough to power your Pi and whatever peripherals you're using.
Anker Portable Charger, 313 Power Bank (PowerCore Slim 10K) 10000mAh Battery Pack with High-Speed PowerIQ Charging Technology and USB-C (Input Only) for iPhone, Samsung Galaxy, and More https://www.amazon.com/dp/B07QXV6N1B/ref=cm_sw_r_cp_apan_i_9R9T3GVDPH9HTN07WNBS
This might work. It's about the same price and has a 5V/2.4A output. Don't worry about frying your Pi, it'll only draw as much current as it needs.
Anker 10000mAh power bank for $20
USB -> 9V converter for $8
One thing to keep in mind, depending on the converter & battery, the current's limited to something like 600mA. More info here.
Anker is known for being good value.
Big 4 for $502
The rest of the gear is relatively cheap, like:
Random stuff from your house / DIY:
Clothes could be a whole post of their own. Start with what you already own and upgrade to wool/synthetics over time.
Food from the grocery store is cheap. Skip the fancy freeze dried stuff. Shout out /r/trailmeals.
Skipping unnecessary items is the best way to save money. Amazon will make you think that a huge knife, lantern, chair, axesaw, and other 'camping' gear is important. Don't let them trick you into wasting money / weight on garbage.
EDIT: added a few items
You can get a power station for not too much. That will charge your phone and small appliances for a while. Harbor freight has some good ones. I’ll post an example at the bottom. You should also have a car battery jumper in your car, will also charge your phone and be a flashlight. And I bought a portable solar panel you can recharge your phone battery packs.
Anker Portable Charger, 313 Power Bank (PowerCore Slim 10K) 10000mAh Battery Pack with High-Speed PowerIQ Charging Technology and USB-C (Input Only) for iPhone, Samsung Galaxy, and More https://smile.amazon.com/dp/B07QXV6N1B/ref=cm_sw_r_cp_api_glt_i_WQWJNTMTXVRVDM47MDC0
[Upgraded]BigBlue 3 USB Ports 28W... https://www.amazon.com/dp/B01EXWCPLC?ref=ppx_pop_mob_ap_share