It's simple, really; your refresh level is the level you start at.
There is no max cap for refresh or cost of powers in the game; Refresh and skills increase with the experiences of your character, major choices he/she/it makes in life and what they teach of the world and about oneself. In short, milestones allow you to rearrange skills, stunts and aspects and the more important ones give skill points and Refresh. The more conflicts, changes and realizations you have, especially those large enough for you to choose to change who you are, the more free will you have, the more impact in the world and yourself. Thus, your refresh increases.
Power has no limitations at all, not even experience or free will. Any character, if they find a way to boost their power, could increase their abilities by adding stunts and powers. For example, a mortal could sacrifice aspects of his life to train harder, focus more, lessen distractions - or compromise his morals, his beliefs, and other important things to him to become better at his job. Any spellcaster could abandon the careful control of the Laws and gain more power through Black Magic far more rapidly than he could with experience, or tap powers from other sources, from human sacrifice, to demons, to chaotic leylines to the power of Faerie. And there are other shortcuts too - magic objects, rituals, pacts with various creatures, curses that change you and so on.
Gaining power that way, you make the choice to use it regardless of the consequences, before you are experienced or wise or disciplined enough to wield it and remain yourself. Thus your refresh does not increase and eventually becomes negative as you turn into a monster without free will.
In short, the game starts out at a set refresh level. The characters have either to earn more power though choices and character growth or simply choose to take it anyway. A spellcaster -from minor talent to wizard- could, within a few minutes, break the Laws about killing, mindraping and transforming several times. In but a single scene, he could get -6 refresh worth of Lawbreakers, increasing all his rolls about such magic by 3. That's nearly doubling your power by becoming a monstrous warlock.
Similarly, that same spellcaster could use his powers over many situations where he's pushed to his limits and learns new things about himself, chooses new ways in his life, and refine his powers over several years, taking Refinement 6x. It's a similar power increase from experience and accomplishment as you could do via dark magic only this time he retains free will and is not a monster.
An example of advancement through experience, choices and adversity is Harry Dresden; he started out at Submerged (10 original refresh) but over the course of the series has so far exceeded 20 refresh worth of Powers while retaining free will.
