You might consider sending an email to the address on the site that NatsAddict gave you. If you can, send a picture of the cash register, with the serial and model number. Ask what the value might be. They give no value for the 4 drawer model they show at the top of the page, and the prices they show on the other registers don't seem to be in the same category.
It wouldn't hurt to send an email and find, especially if you don't tell them where the cash register is right now.