Hitec is running a series of ads in the car magazines that addressed just this issue. It explains what you get when you spend more money and how and why the perfomance increases as well. It is for the customer to ultimately decide if the additional cost is worth it and to some modelers it is not.
In the end what it comes down to is how much it costs the manufacturer to build it, how much they mark it up to the dealer/distributor, then how much the dealer marks it up to the consumer. I guess if no one made any money then they would cost much less... well then no one would be left in business to make or sell them.
$115 gets you a digital, coreless motor w/ heatsink, bushed case top, hardened steel pins and Titanium gears in a servo that produces 333in/oz of torque... at least from Hitec. In the past you had to spend $85 per servo for a digital/coreless servo and needed to use more servos in your model. Now you have better servos and don't need as many... this will actually save you money. Granted this is all based on giant scale applications.
I guess the same could be said about anything... there are all levels of everything; if $115 is too much to spend on a servo there are plenty available for less, but don't expect them to perform as well.
For more info on what you get for your money, go here:
http://www.hitecrcd.com/servotech/performance.htm
Mike.