Do you pay it's casting cost and the overload cost or just the casting cost?
You can pay either, you don't pay both. Overload is an alternate cost, not additional.
Teleportal is overpowered than. I've won every game I've played it, even did 20 damage in 1 turn with it in early game (think 2 cobble brutes, and 4 1/1 goblins)
True. Until you meet someone with {Electrickery} in their deck.
That too is in my deck...