You don't specify if you are strictly looking for a trip to London & Paris, or if that was just the one that caught your eye. Knights & Lights is one of
ABD's most expensive trips (there are trips that *cost* more, but they are much longer trips. On a per-day basis, I think Knights & Lights is close to tops in price per day, if not *the* top). There are lots of other ABD's that are not going to cost you $18,000 for a family of 4. It's still not cheap, but looking at the most expensive trip doesn't give you an accurate picture.
But yes, there are other tour companies out there that are good, reputable companies, that will do London & Paris cheaper than ABD. You're the only one who can decide if what ABD offers is worth the extra money or not.
But I also feel compelled to say, traveling with ABD is not about "getting your Disney fix". The trips are *very* un-Disneyfied. You don't see Mickey & Minnie and Donald at Trafalgar Square. Really, the only nod to Disney is the pins you get on every trip. For ABD, "Disney" is about the *quality* and the access that Disney has. It's not about the Disney characters, etc.
I hope you have a wonderful trip, whoever you end up traveling with!
Sayhello