Please note that these are not an exhaustive list of all medieval equipment available. Simple items, such as pitch, tar, burning oil or defensive fortifications and such can be RPed out.
Siege Engines cannot be bought by Iron Islands or Dorne. As they are impossible to manufacture in the Iron Islands and impossible to use in Dorne.
200g per 10 Unit
Description: The battering ram is a moveable tree trunk tied to a frame, operated by up to twenty men. It's main design is to use the trunk to shatter down wooden doors and stone walls, though it takes a significant amount of time to do so. As it is made of primarily wooden pieces, the top cover of the battering ram uses a canopy of wet fresh hides to prevent any sort of fire attack against it.
500g per 10 Unit
Description: Trebuchets are siege weapons which use a swinging arm to throw a projectile at the enemy. A trebuchet projectile can be almost anything, even debris, corpses, or incendiaries, but is typically a large stone. Dense stone, or even metal, specially worked to be round and smooth, gives the best range and predictability. They are vastly inaccurate, but when thrown in large quantities, effective through volume. As trebuchets are constructed on site trebuchets require a setup period of three posts.
Siege Tower / Belfry
1000g per 10 Unit
Description: A siege tower or breaching tower is a specialized siege engine, constructed to protect assailants and ladders while approaching the defensive walls of a fortification. The tower was often rectangular with four wheels with its height roughly equal to that of the wall or sometimes higher to allow archers to stand on top of the tower and shoot arrows into the fortification. Because the towers were wooden and thus flammable, their front-facing side had to have some non-flammable covering of iron or fresh animal skins. Siege Towers are constructed on site, and require a setup period of five posts.
600g per 10 Unit
Defending Castle or Hall Only
Description: The Scorpion is an artillery weapon that shoots enlarged crossbow bolts. In essence, it is similar to the ballista, but smaller, more compact, and not as capable of destroying walls. It has enough penetrating power to tear into dragon scales. It is not movable and thus is generally placed on stationary defenses such as castles or ships.
Longships and Cogs are the only ships the Ironborn can purchase.
Ironborn Longship (small)
300g per 10 Unit
Description: The small longship is a raiding transport ship built for speed and its ability to navigate the sea in all of its conditions. With a crew of 25 men, two rows of oars provide its main means of propulsion. It is steered by one side rudder, while one lateen sail serve as its secondary means of propulsion. The small longship does not come armed with any weapons, and as such is not meant for ship-to-ship naval warfare.
Ironborn Longship (large)
1200g per 10 Unit
Description: The large longship is a warship built for speed and its ability to navigate the sea in all of its conditions. With a crew of 116 men, two rows of oars provide its main means of propulsion. It is steered by one side rudder, while one lateen sail serve as its secondary means of propulsion. With a stronger and more flexible hull than the small longship, the large longship is tougher overall and less susceptible to ram attacks. The large longship is armed with a battering ram at its prow and one scorpion.
1000g per 10 Unit
Description: The cog is a single-masted round ship, clinker-built with steep sides and a flat bottom. Slow because of their hull shape, round ships had to await favorable winds before sailing from each port of call. However, the increase in carrying capacity made a slower passage economically feasible. It had a rounded bow and stern, and the rudder was located in the center of the stern. Built as a sturdy seagoing trader, the cog could hold a crew of up to 48 men, and cross even the most dangerous passages. Should it be militarized, fore and stern castles could be added, with the added benefit that their high sides made them more difficult to board in naval combat.
1500g per 10 Unit
Description: The chelandion is a medium-sized dromōn type galley with two rows of oars, which provide its main means of propulsion. It is steered by two quarter rudders at the stern, while one lateen sail serve as its secondary means of propulsion. It is armed with one scorpion, fore and stern castles, with the option of adding a boarding-bridge onto its prow at the cost of some of the ship's speed and stability. With a crew of 108 men, the chelandion is a formidable warship. Not suitable for the rough seas.
3000g per 10 Unit
Description: The dromōn is a heavy galley with two rows of oars, which provide its main means of propulsion. As the standard warship in Westeros, it is steered by two quarter rudders at the stern, while two lateen sails serve as its secondary means of propulsion. With a stronger and more flexible hull than the chelandion, the dromōn is tougher overall and less susceptible to ram attacks. Its prow is armed with a battering ram and a boarding-bridge, and castles protect the fore and stern. Due to the ship's large displacement and crew size, its stability and speed are not affected by the boarding-bridge. Armed with a battering ram on its prow, two scorpions, and a crew of 200 men, the dromōn is a versatile and powerful warship. Not suitable for the rough seas.