Let's look at it another way and do some quick math. Suppose your goal is to get to challenger tier. Suppose this would take you 1,000 hours of game play. Suppose we value your time playing LoL at $5/hr. Its costing you $5,000 in time to achieve your goal. Suppose 10 hours of coaching at $15/hr saves you 5% of the time required for you to achieve your goal.
Base Cost ($5,000) + Coaching Cost ($15 * 10hrs) + Time Cost of Coaching Sessions ($5 * 10hrs) - Time Savings (5% * $5,000) = Cost After Coaching ($4,950)
Not only do you personally net savings in this conservative example but you also achieve your goal more rapidly. If your goal truly is to be competitive, every % point counts and is worth pursuing!