Once you start assembling and selling these, you'll quickly realise why many things are priced the way they are.

Especially low-volume tings like "pre-assembled DIY kits"

The BOM already costs more than USD 40 so USD 80 is probably a more realistic number for a preassembled kit.

Would be happy to pay USD 80 for it as well

This. I no longer have access to a soldering iron and want a bunch of these, not enough that it's worth the cost of buying equipment and the time spent making them.

Help us give you money! :-)

You can get good soldering irons for less than $100.

Yeah but I have to buy the soldering iron then spend my time figuring the kit out and doing the job.

I’d be happy to pay OP for their work and everyone wins.

Way too low for that - good rule of thumb is 1.5x to 2.5x the cost of materials for labor if someone else is going to do it for you.