One aspect that doesn't get mentioned too much is the sketch's subject matter. Not necessarily the character (or even the artist), but the scene/moment that's depicted, if that makes any sense.
Take this sketch for example, which sold for over $1100 via Ebay auction recently:
If one isn't familiar with the show, it just looks like a normal Gambit sketch. The artist is Matthew Sutton - very talented, but not a huge name.
There are other Gambit sketches from the X-Men 97 set that are less desirable, and there are also other Matthew Sutton sketches that are readily available.
The unique thing about the above sketch is that it's a direct nod to a beloved scene in the show. Possibly the most revered moment by fans:
"The name's Gambit, mon ami. Remember it!"
The Ebay auction looks to be real, but probably two collectors in a bidding war.