Quality, components, packaging, and warranty are some of the things that will make up price differences. Rebranding with a "famous" brand sticker/etching will also raise the price. This day and age, there are lots, and lots of OEM manufacturers making servos for many different brands. Take Turnigy, for example, they have multiple manufacturers making servos with their name on them. That was last time I looked at servos from Turnigy. I can't stand to navigate HK's website and don't shop there anymore
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The components, mainly the motor and gears, will determine price. Brushed, brushless, and coreless motors...how fast and how much torque do you want? Plastic, composite, copper, brass, aluminum, steel, and titanium gears...what's your needs? All plastic case is fine for some. Some servos have middle section made out of aluminum for improved cooling, and other manufacturers will "bling" out to the extreme with fancy all metal cases.