[quote=@Gisk]
I definitely think you're on the money about cost, although I don't know that we want to plan too much about magic. My idea here is for the magic and fantastical elements to be a little like window dressing for a college town slice of life. 

I like the idea of keeping the process of doing magic deliberately vague, since it's a tool for the narrative, and not the focus of it. But there's definitely a need to define some general rules and boundaries. 

Like you say, any spell has a cost, and the cost must be paid for the spell to remain in effect. By the same token, I think we can hold with "equivalent exchange" as described in Fullmetal Alchemist. 
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Equivalent exchange falls under Hermatic magic, also known as the Laws of Sacrifice - giving up in equal value to what you obtain. Some rituals, like stone circles, taps into people in the circle to charge a magical battery, which can then be used to work more powerful magic. However, it's very important that you replenish the magic you use. There's a magic circle in the series Outlander that transports a woman back in time, for example, at the cost of any gemstone she's wearing. The purity of the materials does affect it.

Sympathetic magic, or the Laws of Sympathy, is basically like calling to like - Voodoo dolls and spinning straw into gold, that sort of thing. If you can establish a relationship with runes and symbols, they work much the same. Like the Wizard's Eye, a drawing of an eye that allows a witch or wizard to see what's going on there.

Often, it's just two different methods to obtain the same result.