General rule of thumb is used parts are approximately 50% of the value of new. This will go up or down depending on demand, and condition of part. Demand for the 4.24 is good, as there are only so many of them that pop up for sale (those that buy them tend to keep them). T-case seems to go for about $100 (low demand), and 1/2 the value of the gears is $325 ($425 total). Given the demand is going to be slightly higher, $550 seems fair.
Bottom line is - it's worth what someone will pay for it. If in doubt, start high, and drop the price until it sells. That's what the for-sale section is for.