SFP+ = low cost for the manufacturer.
The problem is that most manufacturer do zero market research, so they don't seem to understand what there's a demand for. For some reason it also seems like most companies believe that prosumers/SMB users don't want configurable hardware, so they make basic hardware that doesn't really sell in the end...
It's partially the chipset makers fault as well, as they segment their products between consumer and business, which often means the consumer products end up being gimped versions of the business version. Or if the consumer chips has the higher-end features, they're rarely implemented in consume products, as it's too much work from a software perspective.
As for making a low-cost 10Gbps, not going to happen yet. The parts are still too expensive, but hopefully we should see some products this year. Last year at Computex, Realtek was showing off several 10Gbps boards with a combination of Realtek and Aquantia chips. Obviously there was no mention of pricing though. I would expect 5-8 port 10Gbps devices to retail for closer to $300-400 when they launch based on more affordable platforms like Realtek/Aquantia.
It's not always about profit margin, sometimes the parts are simply expensive. If a 10Gbps network card is $100, how can you expect paying less for a switch with 5-8x as many ports? Obviously the Aquantia cards has gone for far less (black friday deal), but unless there's a certain volume of a product being sold, prices are kept higher to make up for the low turn over of product. If you can convince 4 billion people to invest in 10Gbps network cards, I'm sure I can get you a very different price on those cards...
The QNAP products are based on Marvell and there's a teardown here -
https://nascompares.com/2018/04/12/unboxing-the-qsw-1208-8c-qnap-10gbe-switch-its-finally-here/