In most cases, "word-word.com" domains sell for only a small percentage of what the "wordword.com" version would sell for.
The problem with hyphenated domains is that if you verbally tell someone to visit "word-word.com", they will naturally visit "wordword.com" instead... unless you specify that there's a hyphen, which is rather cumbersome to say ("word, hyphen, word, dot com").
In general (there are exceptions), it's a good rule of thumb to stay away from hyphenated domains.