I don't claim to be an expert in the economics of manufacturing but it makes sense that they're more expensive than a normal micro SD card.
Usually, the higher amount of a product that is manufactured the cheaper it is to make it per unit. Therefore because there's probably more micro SD cards made in one day as there are Vita memory cards total, the price of Vita cards are obviously going to be more expensive.
Now the argument on whether or not Sony should have even made Vita's memory proprietary is objective. On one hand it would be awesome if I could get a 128gb micro SD card for my Vita for less than the half the price of it's current 64gb. Mainly because I desperately need one. On the other hand, there's a very very VERY good chance that if using normal micro SDs truely led to piracy the Vita would have been dead in 2013.
Honestly while it sucks that it's memory cards are expensive, I only had to worry about the price once. I'd say that's a better alternative than having the device die in 1 or 2 years due to piracy.
PSN: extremeM
PlayStation Vita Japanese Software Sales (Media Create Physical/ Famitsu Digital)