Reputable OEM component sellers are not going to sell 20 DS1307 breakout boards for $20.
The question was how the low end price is possible. The answer lies in the manufacturing of said products, and
their component sourcing, which is being discussed right now.
A reputable component seller might sell 2000000 DS1307 ICs to a manufacturer who uses it in a mass product, is then left with 20000 parts in excess, and then uses a cheap Shenzhen PCB fab that manufactures and assembles the modules for a few cents each, then sell them on Banggood. All this would usually go through various steps inbetween; you'd either lose traceability or you won't, but for the end customer, a hobbyist buying a cheap module, it doesn't usually matter.
The key thing to understand is that component pricing is arbitrary, and given enough volume and the right contacts, it can
easily go down by 90% from what you see on Digikey for 20pcs, and this doesn't
necessarily mean compromising the traceability. OTOH, almost anything is possible, so this is just speculation. You can't know from the price alone whether they are fakes or not. Some are, some are not.
It's fairly common to see this discussion happening: if an IC costs, say, $5 at qty 10 on Digikey, or $2 at qty @10000 at Digikey, how a finished module can cost "only $1" on Ebay - "it must be fake".
But, this doesn't take into account for a few facts:
* A
typical markup on the Digikey seems to be around 3x, even for large volumes - hence, from directly sourcing from the manufacturer at large qty, it's well possible the "$2" price is down to around 50-70 cents.
* The assumption that a few support components (a few resistors, capacitors, a connector) are expensive is often wrong by x100. Even brand resistors and chipcaps on DK cost something like $0.002 each. In volume, local Chinese brands, it's evel less.
* The assumption that PCB manufacture and assembly is expensive, is clearly wrong. A small module can be fabricated and assembled for a few cents. This is because the highly automated process. A PCB panel can hold thousands of module PCBs because they are so small, and a modern P&P machine can populate large panels at a pace of, say, 1 second per module.
* The shipping costs can be surprisingly low when one ships hundreds of thousands of small packets from China or HK. The real cost happens at the delivery, in the receiving country, and the real cost is spread across the customers of the postal services of the receiving country. This isn't fair, but it's the reality.
So, for a "typical" $1 module that contains a "$2 part", the cost might
actually spread down like this:
* The $2 part, actually, in volumes: $0.50
* Support components: $0.05
* PCB manufacturing: $0.05
* PCB assembly: $0.05
* Testing: $0.00 - no testing
* Packaging: $0.05
* Worldwide shipping: $0.15
* Ebay/Paypal/Ali cost: $0.05
* Profit: $0.10
If a counterfeit part exists, and
if it is significantly cheaper, you can of course push that profit margin up. But, a counterfeit is not always available at all, or the actual manufacturer is actually
competiting on the Chinese market (because that's a huge market opportunity - only working with lawsuits against fakes won't work - it's better to sell your good stuff for good price, compromising on your profit margin!) and selling the genuine chips for affordable price.
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