As with most things, depends on brand, age and condition. I have restored a couple of late 60's early 70's 'stereograms' and the value of them is pretty minimal. That said, you could be sitting on a real gem... I'm not.
My favourite completed project is a GE Diplomat stereogram I bought for $50, spend about $10 on bits and I don't think it would be worth more than $50. It lives in the shed. The other completed project I was given as the person was having a clean up. It cost me about $5 for bulbs and about $20 for a stylus. Even being a nice HMV stereogram cabinet it is probably worth about $50.
The third is a bit older and was given to me as it doesn't work. Being an old valve radio, it is a long term project. Not sure if it will ever be worth too much, but fun for the challenge.
Anyways, have a trawl through a couple of the audiophile type forums. That will give you an idea of the collectability and maybe value. You might have a real gem! Good luck.