Why are master links so expensive?
There are two ways to price a product - Cost Plus or Value Add. To survive, Cost+ must be less than VA or you can only sell your product for less than it costs to make. If C+ is less than VA, you an sell at any price point between them. Too close to VA, you encourage competition, to close to C+, you forgo profit. Something like a master link, I can't imagine someone thinking "I know, I make master links, sell the 10% cheaper and get rich" - the market is too small, and too brand loyal....... So the only competition to master links is not using one...... 

If you don't use a Master link, you need a special tool (chain breaker), and skills to use it. The tool is about 10x the price of a master link - so you could buy the tool and not have to buy a master link, or you could just buy the master link - 10 times if needed. Most of us buy a hand full of master links and just use them as needed till gone, the $25 price tag is forgotten a long before the next $25 top up is needed, but if we run out, that $25 is well remembered reassembling a chain with a tool - as money very well spent...... 

I own a tool, and would not be without it, but always use a master link.........