The cost to build a wooden kayak is:
-same cost as a plastic boat for the full kit
-about 50% of the above cost for resin, fiberglass, epoxy and other materials give or take
-possibly more as some kits do not include seats, rudder, skeg or rigging, some do some don’t.
-tools if you don’t own them (if you work wood then not an issue)
-protective gear because you’ll go through plenty of goggles and masks to avoid breathing in epoxy fumes or sawdust the former which is toxic the latter which causes nasal cavity cancer.
-lots of sandpaper as the biggest part of the project is sanding.
-energy/electricity or battery cost for your sander or power tools
-lighting for your workspace if needed
-heating for your workspace if needed
-loss of use of your workspace you are using for boatbuilding instead of doing something else with it
-80-120ish hours of your time (sometimes less, sometimes more).
Composite boat building companies are able to not only build boats with custom epoxy or fibers (glass, kevlar, carbon) tuned to that specific boat’s purpose, but customize the fiber type or thickness based on the area on that boat making it thicker in certain areas or more porous in others for more epoxy or less porous for weight savings and less epoxy in others. They also buy in bulk so wholesale market is cheaper for them or just make it themselves. They also are able to build abroad as Stellar does in China for cheap labor though Trump laws have affected that somewhat. Just go to West Marine and gawk at the retail prices of everything!
When you come down to it your first boat or three might not be the weight or stiffness or smoothness or speed or handling you expected since you have to know what you’re doing not just in working wood but boat building itself. This is also a risk not just that puts how nice (or disappointing) it will be for your enjoyment but that makes profit impossible. The market knows a great looking wooden boat could be a botch-job because unless you are a well known boat builder with extensive reputation, when you turn around and sell the boat, the market will demand a price at somewhat of a loss as compared to all the materials in the first section of this message to account for this risk. Without that reputation you will never earn a penny for the time it takes to build one.
I had a friend build me a wood boat a few years ago that was his design and paid for construction materials only. Thankfully I did not pay him for his time because I realized he was a yacht or boat architect and not a kayak builder who built just that: a yacht that looked like a kayak. The two have very different characteristics. It sucked, it was a heavy piece of junk, and in addition it had a very fragile finish. But unlike a well built wood boat that is 30-40lbs for singles, or 50-60ish lbs for a double and easy to carry in one or two people (respectively) and be careful with it, a 180lb behemoth with an outrigger is impossible to handle carefully which makes the wood/composite design suffer more trauma than is good for it or your wallet. Plus it was too heavy to get out of its own way. And unlike composites if you get wood wet with a hull puncture it’s a problem. Well it’s always a problem but 100x for wood because it will rot and gets expensive to fix.
I think most people build wood boats out of pleasure for building it. If you want to save a lot of money buy used plastic. If you want to save some money to get something easy to handle on or off the water, buy used composite in a material that is congruent with your budget and intended use. When it comes down to it wood is only for show and enjoyment of the build. For the risk you take and aggravation you go through the new composites are so good and not that expensive that it seems like a foolish economy to DIY unless you like the process.