As an electronics engineer:
Small batch (10 or so) fully integrated and assembled PCBs of the type of Airgradient without testing, without a case, and without any certifications will run you probably around $50 per piece. That is without any development time, no 3D modeling time to build a case, no software, engineering hours, testing, etc.. Just raw BOM cost.
Boards are cheap, assembly service at low quantities is expensive. Making 10 PCBs by hand is also an option, but at a low $50 hourly rate, it would probably take 6-8 hours to hand assemble if you factor in 10% board mistakes with hand assembly. That brings you to $80ish per piece with no software or case.
The other way you can do this is hiring out very low income countries through fixed-rate contracts via fiverr or Upwork. But you will maybe have to shell out around $1000 to get people to actually take the contract, in which case it is only 30% cheaper than the airgradiant kit for vastlt lower build quality.
Much easier to work with modules and jumper wires, test it yourself, and solder when needed. Soldering station and supplies is $50 total and with modules you can probably get it done a bit roughly for $50 per piece total, but the benefit ia your free time costs nothing money-wise and you learn a lot.
Yes you can do things yourself for much cheaper than commercial products. The keyword is yourself. You often times can't hire someone else to do it for cheaper than a commercial product (with exceptions).