Three levels, paid at different times
Metadata is the standing cost of having a skill installed at all. Instructions are the cost of using it on a task. Resources are the cost of the specific file the task reached for. Nothing warns you when a skill is too heavy, so the budget is only visible if you go looking for it.
Twenty skills is not twenty skills of context
At roughly 100 tokens of metadata each, a shelf of twenty installed skills costs on the order of two thousand tokens whether or not you use any of them. That is small. The real cost of over installing is not context, it is that overlapping descriptions make the wrong skill win.
Keep SKILL.md short by moving detail down
The working rule is to keep SKILL.md under 500 lines and push the long material into references/, which is loaded only on demand. A skill that inlines its entire manual pays that whole manual on every activation, including the activations where the agent needed one paragraph of it.