When thinking about game design, one issue that comes up is how to handle the cost of skills with little or no game use. The traditional approach has been to make them cost as much as useful skills, which is clearly a bad idea and hoses characters with interesting character conceptions.
Although technically one might expect minor skills like “mason” to have some small point value, I quite respect D&D’s choice not to implement this. There are a number of practical difficulties in assigning a point value to unimportant skills:
1. It takes space in the rules.
2. Since D&D is choice-based rather than point-based, there is no mechanism by which a skill could cost very little to get.
3. If a skill is worthless, it is cheap to become incredibly good at the skill. There is a risk of players then trying to abuse the skill by claiming they can do incredible, useful things with this much skill.
4. If the GM does not adjust the skill cost to the campaign, it would be all to easy for some of these cheap skill to become very underpriced because they are used much more often in that particular campaign.