Hey Elanhil,
I don't get around to Holy Worlds very often, but I had a few minutes today and your post caught my eye.
A couple of questions...why is gunpowder expensive and hard to find? Have you researched the components of it? The basic building blocks of black powder are organic and very common (for instance, the Potassium Nitrate needed for saltpeter can be generated with plant matter, dung, and urine composted in the right manner. Let's face it, most cultures have no shortage of those items!

) The fact that the compound might be expensive also doesn't make it exempt from being used in war. If a kingdom wants to protect itself (or has imperialistic designs on another) they are going to utilize the best weapon for the job, and sad to say, guns win over everything else in that capacity. Perhaps your world hasn't developed the right technology to produce well-functioning firearms...I would buy that explanation for why they aren't used in war before I'd be willing to accept the "expensive" line. (Not to mention, war=expensive is a pretty widely accepted equation.)
Another item that stood out was your mention of commoners not riding horseback, but rather using buggies or carts. Unless there is some situation in your region that eliminates all of the tan-able hide bearing creatures (cows, horses, deer, etc.) I would not think that a leather saddle would be pricier than a cart or buggy. If you go by today's standards, a horse drawn vehicle is around 10 times as expensive as a saddle. Now, if the issue is the price of an animal to ride, that I might accept more readily, but then you have to change the information to say that commoners typically walk. (Which, by the way, is historically pretty accurate.)
So, those were the items that stood out to me as perhaps needing some rethinking. Nice work, though, in tackling some of the nuances of your world...details that will give it depth as you flesh them out.
Blessings on your endeavors!
Becky Minor