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.