@Doctor G
Please be more specific and state which pen kits in particular you have been looking at ... please give name of kit and manufacturer.
When you are considering costs, please be aware of the packaging costs, distribution costs, and other similar costs.
The size of the market would also be a factor in amortizing the fixed costs of designing and tooling. . It's a business not at all like automotive manufacturing (as one "gross" example) because nearly everybody needs a vehicle, but everybody could do without a hand-crafted pen, what with free pens being given out by insurance agencies and the like.
For any particular kit, there may a relatively small market, but that market has to support all the costs, some of which we don't think about.