What is the original Voltron set worth?

Depends on what version you have, its condition, whether or not it's still boxed, etc. As a general rule, the original 80's versions, even in the worst condition will fetch $50 to $75 for the whole set.

As far as the higher value ones, the Panosh Place plastic collection will go for the highest. This is because of the amount of toys they produced. The castle alone, even if not complete will go for $100 or more easily. A new in box castle will fetch anywhere between $300 to $500. Same with the figures and lions/vehicles. The loose toys can go for $100 for the set, with up to $300 for a set new in boxes. The figures usually don't go for very much, roughly $20 each, up to $50 if unopened in original packaging.

You can easily fetch over $1,000 if you had every Panosh Place item in its original packaging, and over $200 if you have the die cast Matchbox Voltron new in box. Hope this helps, and hang on to your Voltron collection, its value will only grow, as World Events Productions keeps pushing to get Voltron back to the mainstream like it was in 1984.