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50 PCBs for $10 might be achievable with large batch size. But impossible with components.
I have soldered a batch of 50 before. DD drivers with FETs, they ended up in the 2-3 $US region if shipping was free (arrow as an example is no longer free below 50$ afaik). boards came from OSHpark and I looked a very long time to get the cheapest components (across multiple platforms, countries and more).
Also your understanding of buck drivers is not very good. 80% efficiency for a 2A driver is really sad. Well-chosen components should seen 90-95% without a lot of difficulty. As battery voltage tapers, duty cycle increases to the point it basically becomes direct-drive so both efficiency and utilization of the bottom end of the discharge curve will be better than AMCs.
PCB is not the problem you can get 250 17mm or 160 20-22mm PCBs without gold plating for like 12-15$ shipped
but there are more costs to be efficient
stencil like 10-15$ plus shipping
so lets say like 25$ for stencil and shipping in best case
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So a quick calculation for the cheapest DD+AMC driver
—>0.1$ per PCB
0.1$—>0.2$ 1 AMC 0.10$ if you buy like 300 with shipping
0.2$—>0.5$ Attiny13a for 30$ shipped
0.5$—>0.9$ any reasonable good MOSFET is at least 0.4$ shipped
0.9—>1$ 5 resistors and 2 capacitors for TA low pass driver design eliminating spikes from MCU
only if you got those stocked, shipping can get this number up quite a lot
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Lets look on the cheap Buck drivers
—>0.16$ per PCB
—>0.31$ decent input and output capacitors
—>1.06$ any reasonable good buck CV driver chip
—>1.46$ switching MOSFET
—>1.76$ reasonable current sense resistor
—>2.06$ Attiny13A
—>2.36$ a cheap Chinese Inductor
—>2.46$ MCU resistors and caps
—>2.96$ reasonable good zero drift OPA and parts
—>3.26$ reasonable good 2.8V OPA to get the Attiny13A PWM low pass DC transformation decoupled from the Battery voltage
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My series of Buck drivers related to LM3409 30mm size example with 19% tax and free shipping
0.30$ PCB+Stencil (even if its now like the 4. PCB revision with improvements and I have thrown away previous PCBs a lot, counting that more like 1$)
0.85$ input and output caps
1.90$ LM3409
2.60$ TI switching MOSFET
3.00$ 2 current sense resistors (1W 1% 200ppm 667-ERJ-8CWFR047)
4.05$ Attiny85 (bulk of 100)
4.90$ Bourns Inductor (bulk of 50)
5.10$ MCU resistors and caps
6.10$ TI TPS709 LDO
The problem is here you have to invest quite some time to maker the PCB design right, place all components on the first prototype and hope it works
BTW you can buy solder it yourself kits from me if you request
I suspect the market for unsoldered driver kits is too small to be worth the hassle for manufacturers. However, I think there might be potential for a BLF group buy.
There are lots of driver designs shared by BLF members on OSH Park that probably all use fairly similar components and can run open source firmware. The Texas Avenger TA series in particular has a huge number of variations based on the same basic design.
I’ve been wanting to assemble my own drivers for some time, but I haven’t got around to ordering all of the components. Many component suppliers have either minimum order amounts or fairly high shipping costs and if you can’t get everything from one supplier it’s even more of a hassle.
I wonder whether there would be interest in a BLF driver ‘starter kit’ that contained all of the components necessary to build a certain number of drivers?
back when I started to look for these “kits” I could never find one. we should have some kind of poll or survey to figure out how many have the equipment, interest and need to purchase such a kit (and which drivers would be in demand)
Getting started with the components is actually a lot easier than I recalled.
In the original TA series driver thread Texas_Ace provided a link to a DigiKey shopping cart with all of the necessary components apart from the 7135s. The only item currently unavailable is a resistor and there are three alternatives available. If you’re in the US shipping isn’t expensive and it’s free for international orders over $50.
You can get 100 7135s for $12.58 shipped from FastTech, although I’m not sure whether they’re the good ones.
The 17mm TA driver PCBs are $2.25 for three.
Ordering 25 each of the ATtiny85, the Schottky diode, and the cheaper FET, 50 each of the capacitors, and 100 each of the resistors comes to $71.27 with free shipping. Add the 7135s from FastTech and 17mm boards from OSH Park and it works out to ~$4.10 each for your first 25 drivers, with lots of the inexpensive components left over for your next batch.
When I first looked into it I probably baulked at spending over 120 AUD on driver components, but these days it seems quite reasonable. I guess it just depends how far down the rabbit hole you are. It helps that I now have a decent soldering iron, hot air station, adjustable power supply etc. 
Of course, unless you’re doing the soldering for fun, you’re probably better off ordering an assembled driver from someone like Lexel or Led4power.