Quoted from robin:
They seem to be getting more rare, especially ones that don't have playfield damage in front of the cabinet.
When X-Files came out games in the field started to wear very badly in front of the cabinet due to air balls caused by the strong magnet under the playfield actually damaging the wood. Sega upon request supplied owners one of two free kits to cover up the damage and prevent future damage. Kit one was supplied for later production games (which already had less powerful magnets under the playfield than earlier games) and included a thin metal plate (made out of material similar to the plates at the entrances of plastic ramps) with self-adhesive on one side so it would stick to the playfield, and a clear playfield plastic piece to mount above the metal plate to posts across the area directly in front of the cabinet (to prevent air balls in that area). Kit two included both parts from kit one plus a new rom to lower the magnet force on early production games (you were requested to return the original rom).
My buddy and I bought X-Files NIB at a close-out price of $2195 each in 1997. Even though we got them just a week apart, my buddy's game was early production (with the stronger magnet) while mine was later production. Neither of us has ever installed the cover-up kit on our games. Keeping the playfield waxed on a HUO game with the later production magnet setting has resulted in no visible wear whatsoever. And the game looks so much better with the original playfield unobstructed. My buddy went underneath his X-Files playfield and backed the magnet out a few turns (which lessened the magnetic force topside) and he has had no wear issues with his HUO game as well.
Both of our games are in great shape, with no cabinet wear and no operating issues. The game is tough because the ramp shots are very tight. Some but not all X-Files I've seen allow making the right ramp off a backhand from the right flipper, which makes it a bit easier, as repeatedly making ramp shots is the key to progressing through the various modes.