A small cannon will cost you 2000, and a single ball and shot (aka the powder needed to propel it) is 10 GP together. A small cannon does 2d8+dex damage and does double damage to objects and structures, has a range of 200/600, and requires an action to load and an action to shoot (effectively a two-man crew). It weighs about 100lbs and it five feet long.

A regular cannon is 4000, same cost for ammo, but thanks to a more advance design and larger bore it does 4d8 damage and has a range of 500/1500. Those weigh 400 lbs and requires an action to load, shoot, and aim if the cannon isn't aligned to the target (This is usually irreverent for ship-to-ship combat, but important against sea monsters). They are usually ten feet long.

Monster Cannons are 10000 and have their own ammo which is 50GP a pop. These massive monsters are generally not even on ships, but part of a wall, and most ships that do have them are pretty much just the cannon and it's crew. They can do up to 10d8 points of damage with an effective range of 1000/2000, and are a little under a ton (specifically 1800 lbs). Action to load, action to shoot, action to aim.

Regular and Monster cannons can also use unique rounds such as grapeshot (does a weaker cone), Chain shot (effectively a line), or even other exotic types of ammo, though obviously these are more expensive than regular shot. Small cannons don't have the sheer bore to support these exotic types of ammo.

Not to mention, there are practically no pirate cannons smiths. Learning how to craft and maintain such weapons are often a government or at least guild secret, and the only times pirates even get cannons is if they plunder them off military vessels, which can happen from time-to-time. At the moment the only nations that employ and manufacture cannons is Doros, Xiadan, and the Empire of Ferric. Only Doros has cannons manufactured and sold by official guilds, allowing nonmilitary to buy them. Ferric has the best cannons in terms of damage and range, Xiadan's are the only nation thus far to be able to reliably use magic with their cannons, but Doros ships often have small cannons on any vessel worth protecting (Incidentally they are also the most common type of ships plundered for their cannons).