I don't have details for 15th century guns, but we discussed guns c. 1800 here: https://old.reddit.com/r/AskHistorians/comments/960r2e/im_a_gunsmith_in_the_early_1600s_and_i_got_myself/ For your guild of 30, the same production rate would translate to 5 guns per day.

But as u/Bodark43 wrote in that discussion, those production rates might involve buying some pre-made parts (locks, barrels). If all of the parts have to be made by the 30 guild members, then a production rate of 1-2 per day might be reasonable. For guns with a very simple lock, such as a serpentine, perhaps 2.

The final cost will depend mostly on the labour costs. Raw materials will be iron and wood.

When we're speaking of earliest hand-held firearms (early 14th century), it is necessary to remember that they were produced in two very different ways, usuing vastly different technologies - bronze casting or iron forging. This is doubly important if your setting is not based on medieval Europe, as you could easily incorporate equivalents of early Chinese firearms, that were almost exclusively cast.

Earliest hand cannons were usually cast of bronze, an alloy comprising of copper and tin (usually in 9:1 ratio) or red brass (gunmetal), that is a type of bronze consisting of significant addition of zinc and lead (usually around 5% each, replacing part of copper in the composition). Such alloys are easy to melt and cast, as it requires temperature around 1100 degrees Celsius (2000 degrees Fahrenheit). First hand cannons (sometimes called 'gonnes' or 'handgonnes', but this is a spelling variant of a word '(hand)gun' rather than a name proper) were rather small and resembled sleeves 12-20 inches long and 2-3 inches in diameter, with a caliber of approximately 0.6-1 inch.

To cast the firearm, one needs to make a double clay form, much like in the case of casting any open, hollow object (e.g. bell). This means that the mold of the firing channel (a long clay rod) was fired and then was put inside fired mold of the gun itself. Then the mold was covered in sand and the molten brass was poured inside. The whole process was not that time consuming, but you can count one day for making the forms and another for casting process, as you need to wait until both the forms and ready product cool down. For this process you would need a bellfounder (the process is basically identical to the one of bellmaking) and a moldmaker. It can be job for one person posessing both skills (actual guilds tended to be very narrowly specialised, but in fictional world this may be different). At least one assistant to maintain the kiln and forge where the metal is melted would also be of great help.

Another technology is iron forging. To create such weapon, one needs to take a billet of iron, hammer it down to a metal sheet of required length and thickness roughly corresponding to the one of finished barrel. It should be noted that the thickness usually is not uniform, as most handguns were tapered, with thicker walls around powder chamber to provide additional protection against rupturing of the barrel. This is a time-consuming process and preparing the sheet to work with will take at least one day and three to four people: two smiths (long items require assist in holding) and two assistants (usually students or apprentices) working the bellows and shoveling the charcoal.

When the sheet is ready, a second stage can commence. Metal sheet is first made into U-shape on the swage block, then formed into a pipe around a steel mandrel and finally forge-welded to close the seam. The end would be either hammered down, folded and shaped or plugged with a piece of metal and forge-welded. This process requires very high temperature for a weld to be sound, so it takes a lot of reheats, requiring at least another day per item with the same personnel working on it. Access to mechanised tools (water hammer, bellows operated by a water wheel or maybe a golem - this is DnD setting after all) may speed things up, but it will still require two people to hold the long barrel and someone to maintain the level of coal in the forge.

In both cases, the wooden parts and serpentine lever and flashpan (if fitted) would take the same amount of time to complete. Given that we're speaking of well-equipped workshop, we can assume that raw materials and tools are available and the process includes sawing, planing and chiseling the wooden block to size. Also, please note that earliest handguns lacked the stock altogether - the gun was simply socketed onto a short pole, much like the blade of a spear. In most time-consuming variant we can safely assume that a single stock can be made in a few hours, or one per day per craftsman assigned. Forging a serpentine lever takes about an hour, give or take (again, I assume that the workshop has a stock of billets and bars).

As for the cost of the materials, I'm afraid I can't offer much help. Generally, you would need around 5 pounds of brass or 4 pounds of iron to make an early handgun (or 2 pounds if you want to make a smaller one, no longer than 12 inches). When it comes to iron in early 15th century England, one can assume a 1:350 or 1:400 of a silver to iron ratio. In other word, said 4 pounds of iron could be bought for 5 grams of silver that amounts to about 3 pennies (1/4 of a shilling or a farthing if you will). Of course the real prices were very dependent on the availability of resources and local demand so everything I write here is a very liberal approximation, using England in 1420s as a rough baseline. A daily wage for a skilled craftsman was in the ballpark of 5 pence. So, assuming you'd need 3 craftsmen and 2 assistants (let's assume they are hired labourers, paid 1 pence a day), the materials plus work would cost somewhere around 36 pence or 3 shillings (60 grams of silver, give or take). Regretfully, I don't have any information concerning the price of copper in 15th century, so I can't be of much help here.