This is only my opinion, and I'm no expert in this matter but here goes:
Price can be justified by:
The MicroSD as you mentioned can fit anywhere with the right adapter, but at the same time it tends to be more expensive. In general the smaller/lighter the item the more expensive they are (laptop vs ultrabook, chunky USB vs sleek slim USB, SD vs microSD)
Class 10 denotes the minimum speed that the manufacturer has to meet to be able to use the branding of "Class 10" on the card. Manufacturer like Sandisk sells multiple Class 10 cards at different price as they claim it does different speed (eg. Extreme vs Extreme Pro - 45MB/s vs 90MB/s)
Some devices are inherently small, and therefore need a smaller solution. One example would be mobile phone. It would be harder for the manufacturer to try to cram a SD card slot on a considerably small mobile phone, but they can cram a MicroSD card slot (or in some cases some phone don't even have MicroSD card slot at all).
Bigger devices (Camera, Camcorder, or older bigger mobile phone) have the physical space to allow for SD card (or even CompactFlash card), so they don't have to do further R&D to make their device smaller, and simply use the available space.
The MicroSD may (can't say for sure) cost more to produce in comparison to the bigger SD family.
You might go "wait, the smaller the item, the less material they need, the cheaper they
should be then?"
I'd say yes normally that is the case, but the miniaturisation of the SD card means the SD card is a whole more smaller, and complex to built. Yes the material used are less, but the manufacturing process will be more difficult than the bigger sized SD card. Hence why even there are using less material, they can cost more.
Also actually we may ends up with more material required to built the SD card. First you need to build the MicroSD card (with the additional complexity), and then you add the MicroSD to SD Converter. Add up those 2 materials you may ends up using more than simply making 1 SD card.
Also now since we have 2 pieces of item (the MicroSD and Converter card), the manufacturer have another point of possible faulty. It used to be "ok faulty card, replace". Now "is it faulty card, or faulty converter?"
I feel SD card is like the Dot Matrix printer.. they won't die anytime soon.