I've recently bought hi-o.com for $9. I think it's worth about $17
h-i-o.com would have been much better, because that could make 'hole in one'.
I can only find one comparable sale (3 letter + 1 hyphen) and that was for dd-i.com which was sold for $95 at Afternic.
If it's a common abbreviation, then perhaps it might be worth something. Otherwise I would leave those domains to people who buy out of curiosity (like me).