If this piano has been tuned regularly and in great condition, with original matching bench, is is most likely worth anywhere from 600 - 1000 dollars. Depending on your local market, the price may vary.
Well, you have to find out the exact brand name. Depending on brand name, it might cost more or less. But 100$ is a sufficient amount to pay and you can ensure it works.
The cable piano is not a really big seller right now. As of the pricing, they are so in and out of bounds that no one can keep a average pricing on them. Online, the lowest price was 200 and the highest was 2000.
see the serial number of piano and check Brand name, since there are many manufacturers. Around $90 to 100, you can get piano and inspect it will hold a tune.