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A good condition 512K seems to go in the $300 to $500 range. A good 128K seems to go for around $2000. This is looking at a very sparse range of completed listings on eBay.

The price fluctuates a lot on the condition of all the components and buyer interest at the time of sale. Simple problems that require opening it up and doing a little cleaning, lubricating or replacing a bad capacitor can net you a great deal.
 
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