99,000 coins.
99k
99k if u have 99 in that skill.
The skill cape costs 99,000 GP.
you will need a total off 7,842 exp in that skill.
You should have around 737,627 Exp by the time you are level 70 in a skill.
There are farming calculators out there, and depending on your level and what you will be using to level this skill up you can get the price of that item and times it by how many you require for this answer.
463,885exp
725m 725m
$15.00
around 100 gp
2Mill
Around 2k