Clearly, the price of the mark is going to be one of two things, or both.

One, you’ll have to kill Sam. ( Which you might mange to find a way around, as you guys always do).

Or

Two, you’ll become a demon.

Or both.

Or you might lose your soul.

Or all of the above

I just thought of something else. What if the mark gives Dean some of Cain’s powers? Cain said that mark made him the source of the first blade’s power. Clearly it means more than just the ability to use the blade, and now Dean has the mark. He is now the source of the power.

Either way, Crowley already knows, for sure. But ill bet we wont know until the season finale. Cain calls Dean to kill him, and just as Dean thrusts the first blade towards Cain’s chest, Cain says “i didnt tell you the cost of the mark…” and as the blade plunges into his chest, he squeezes it out “you have to kill sam”. As Cain expires, we see Dean’s look of horror as dramatic music plays and the scren cuts to black.