That’s a great question and I’m glad you asked. The one thing I’ll say, I too was very suspicious of $10 spools(more like $10.99 spools) but don’t worry about taking a risk on Amazon. I’ve been refunded for 3 spools in the last 90 days alone that I found unacceptable. Their return policy is there for a reason.
Here’s my list below. I’ve deliberately stayed away from all of those vendors you mentioned because in my experience, you’re paying 20-30% more for just the name. Luckily I kept one-each of all my spools and wrote notes on them. What I learned when I purchased off-brand filament is that it became very obvious by both the outer carton, the spool, the production label on the spool, the vacuum seal and even the damned desiccant bag, that the filament came from the exact same factory in China that Poly, Overture and eSun uses. In other words, we’re all being played as suckers and maybe for the casual once a month buyer, they’re a safe bet, but if you’re going through more than 3-6 spools a month like me, that ends up adding up to an extra spool for ever 4th or 5th purchase.
This is a great lesson in consumerism and buying wisely.
So to answer your question here are some examples and some receipts for the more unbelievable deals. I’ve tried to provide links as well since YMMV in your country or when you purchase. It should also be noted that I view shopping as a competitive sport. 
I’m only focusing on PLA here. I’ve been doing a lot of PC and PETG prints of late so my PLA usage has fallen off dramatically in the last 60 days.
Note1:
Some of these were flash deals that can be here one day and gone the next. But I keep meticulous notes and maintain a list of past filament performance so that if a flash deal comes to my phone and it’s a filament I have a known experience with, I’ll be able to gauge whether it’s a good deal or not.
Note2:
This list is what I paid back when it purchased it. In case nobody has noticed, PLA prices on Amazon have fluctuated dramatically since June by anywhere from 40-60%.
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Colido $14 - Not $10 but still my consistent go-to source for black filament of choice when there are no other deals to be had. a.co/d/ezzYkIN
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XYZ Printing - While not $10, it’s worth mentioning because thanks to Bambu being out of stock of black filament(at $28 at the time), I was forced to by this filament at $18 which at the time felt like a deal given Bambu’s egregious freight and delivery issues. This is the first filament that got me to buy from a source other than Bambu and essentially changed my buying patterns permanently once I got over my fears of getting ripped off. a.co/d/0chY8On
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Ketrum $10.99 - A Total Disaster, stringy, poor adhesion and 2.47% water. Had this been my first off-brand purchase, I likely would have been soured by the whole experience and run back to mamma Bambu. I got full refund without being required to return unused portion, which shows you the power of buying through Amazon as a Prime member. a.co/d/5achiYb
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Jayo $10.39 - My “personal best” in the field of competitive deal hunting.
This was a flash deal that surprisingly has been a good filament and I was suspicious after my Ketrum disaster but armed with the knowledge I could return it, I took the plunge. However, as I was doing the screengrab for this post it went back up to $14 and a few minutes later went to $16. So it proves you have to know what you’re buying and be quick. If this comes back up at $10.39, since black is my go-to filament, I’ll likely buy 4 spools at once. a.co/d/jbomGBN
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Honorable mention, PETG for under $15 - This was a flash deal and an impulse buy but has really proven to be a great find. Now it’s permanently under $15 it seams. a.co/d/dR1OEel
There are other examples but this post is already too long to include them.
Conclusion
Let’s be candid. If anyone is reading this post, that means they could afford an expensive printer so a savings of $2-$20 may not seem like much, but what if you were asked, “Every 3-4 spools you’ll get a free one” then would you change your buying pattern? Also, if you’re running a business, you’ll likely prioritize supplier consistency over price. However, for me, this is a hobby not a profession so I need to be mindful of not over-paying. I clearly don’t mind spending, I just don’t like knowing I got ripped off, and at the moment, our friends at Bambu feel like they take advantage of the uninformed buyer. If this post helps just one person buy more wisely, then it was a good deed.