ORIGINAL: Wild YFZ 450
There is no servo for 300-400 dollars... Majority of good servos are around 100... The most expensive is the new JR's which i believe are 150ish...
Actually Hitec has a line of industry grade servos that can be as much as $500.00, they are larger than normal size and most run off 12V instead of 5. They are basically super-sized version of the standard size RC servos.