I think how easy a skill is dosent depends on the cash spended but on the cash/time spended. Like slayer it takes food, potions, armor, weapon + req a lot of time. Also rcing you must buy around 1m P ess(140m in cash) + req a lot of time so rcing is considered the hardest. meanwhile firemaking you buy Maples or willows(2m-4m)+ around 1 month. Cooking 100k lobsters (30m but the easiest method is buying and resell making i around 5m wasted) + 2 weeks.