We have been told that mystics like Dr. Strange use three sources of power: 1.-Their own mental power (hypnotism, astral projection), 2.-channeling of environmental magic energy (teleportation, energy bolts) and 3.-borrowing power from extradimensional entities (Crimson Bands of Cyttorak, Shield of Seraphim, Flames of Faltine...etc.).
But I have never found any source explaining what these entities get in exchange for lending their power to mystics; I mean, Cyttorak, Sattanish, Dormmamu...etc., don't strike me as creatures that would lend their power for free...
Also, the Marvel wiki says that Dr. Strange's energy bolts can destroy planets and moons... has he ever done so? That looks WAY above what I remember...