ST:TNG Season 7 sold about 80,000 units during its first year in 2003 at the $100 Price point, generating about $8.3 million. I imagine the total is much more now.
No other Trek series DVD was in the top 50 sellers for TV DVDs in 2003. X-Files wasn't there either.
Buffy the Vampire Slayer dvd sets were huge that year. Season 5 sold twice as much units as TNG and made about as much money, but keep in mind the Buffy sets were cheaper.
"Family Guy," "The Simpsons," "Friends," and "Sex and the City" sold even better.