Hopefully my naïveté doesn't shine too brightly by asking this, but why is a good riding mule so much more? Hoping to someday add either a draft cross mule to the family or draft cross horse to the family. I personally prefer mules, which is why I ask! I've seen a few draft mules in my area for sale, usually the same price as a normal draft.
In most areas, mules just aren't as common as horses and, for some reason, not as many people take into account conformation of breeding stock when they're breeding for mules. Most of the mule breeders I've know in my area will take just about any old jack and breed it to just about any old mare in hopes of getting a nice mule. 9 times out of 10, the end up with something that has the flat back of a donkey, a big ugly head, and jacked up legs LOL.
The other part of it is that not every good horse trainer makes a good mule trainer. Because you have to approach them differently, there are a lot fewer good mule trainers so finding one with good training is already hard. Because the supply of well built mules with good training is so small, then any demand at all will drive prices up astronomically.
A few years ago we had a draft mule that lost his mate to west nile. Because we didn't have another to replace him to make another team, we decided to break him to ride. He had just a tiny little bit over 30 days under saddle when we sold him for about $5K.
The lady that bought him said that she'd been searching for years
for a decent draft sized saddle mule and had absolutely zero luck. Everything she came across either wasn't broke at all or had serious issues to deal with and was still in the thousands of dollars