Why do DVD copies of normal network shows cost approximately $30.00
(friends, Cheers...)for a season, while copies of science fiction
shows (Star Trek, Farscape...)cost upwards of $100.00 a season?

As a Trekker from way back, I speculate that the main reason is that
this is what the market will bear. Science fiction fans tend to be
more obsessive about owning their favorite shows, and are willing to
pay more.

Also, note that most sitcoms are half-hour shows, while most science
fiction series are one-hour shows. If the number of episodes are the
same, the SF shows will presumably occupy twice as much disk space.

...And in many cases, an order of magnitude more brain space. :)

In addition to the factors mentioned in the other comments, you
probably also have to keep in mind whatever DVD extras are prepared.
For instance, the various Star Trek DVD releases include many more
bonus features (interviews, new-and-archival making-of featurettes,
etc.) than, say, the Cheers DVD sets. That's gotta increase the
DVD-makers' costs, of course, which may contribute to the ultimate
suggested retail pricing.