In the UK and most other countries there is often hardly any difference between the price of an mSATA SSD and the equivalent 2.5" model.
The example you gave was a used 2.5" drive vs a brand new mSATA drive and the models are not the same. You need to compare like-for-like. For instance, the Crucial MX200 is currently a few cents cheaper for the mSATA version than the 2.5" SATA version on Amazon.
SSD's have steadily been coming down in price and yet at the same time they have been continued to improve in terms of speed and endurance.
There aren't a lot of companies that produce the raw NAND memory that goes inside the SSD's due to the complexity of manufacturing it. It is a fast moving field, there are lots of research and development costs and occasionally issues with yield. Sometimes bigger companies like Micron can buy out their competitors which further reduces competition.
As the technology continues to mature and SSD's become standard in mass produced PC's from companies like Dell and Lenovo then hopefully the costs to buy them separately will be further reduced.