The Hardest Skill Cape?

I would count making the cash part of buyables, which still makes them hard for me. But they are not the hardest as making the money is generally fun, it just takes awhile.

I agree with this, you have to account for the cash making process because without the cash you cannot buy the skill. So skills like prayer and herblore are pretty close to the same level as something like mining in my opinion. Albeit the cash making might be more fun to some people because they boss hunt or merch the money, some might not find it fun though if they are not a boss hunter and do something like rc or wcing for the cash. Unless they find those skills fun of course :p