Guild of Shoemakers and Leatherworkers
The Guild of Shoemakers, Cobblers and Leatherworkers (in its full redundancy) of Ankh-Morpork apparently maintains very tight control over the price of boots, at least. The cost of a minimal-quality pair of boots good for one year's ill-fitting wear is mentioned as AM$10, rising to $50 for a really good pair. The ten-dollar price corresponds to ten days' wages for an average worker (say a Watchman or junior skilled tradesman). In modern Roundworld society (disregarding even cheaper plastic options) a cheap pair of leather shoes or boots can probably be had for one day's pay. When a dollar a day was a common rate in North America, cheap shoes were about $2 and good ones about $5, as recorded in a common song lyric. Boots, then, seem to cost at least five times what they're "worth", comparatively.
The supply of leather shouldn't be a problem. We hear elsewhere that the Cattle Market supplies various tanneries with hundreds of hides per day. How can the price of footwear be maintained at such a premium? We can't imagine the Patrician imposing duties or restrictions on shoes to support gouging by one guild (especially one whose leader proposed to usurp his position as he lay injured). Other prices in the city tend to follow a reasonable model for an early-industrial society (see Cost of Living in Ankh-Morpork) but the lower classes must go barefoot a lot. We're sure that the guild has a new president since the events of The Truth; perhaps under new leadership they'll see the benefits of mass markets created by lower prices.