Electric skateboards typically go for at least $600 (USD). But with his maker skills Instructables member comsa42 was able to convert a normal skateboard for around $350. That said, his project will give you an idea why these things cost a lot.
Comsa42’s electric skateboard goes up to ~19mph thanks to its 280kv RC brushless motor. Its pair of 5000mAh batteries let him go up to 6.2mi per charge. He also made an app to control the motor via Bluetooth. I’d say assembling the hardware is easy, but then again you need to weld the motor mount to one of the trucks. You also have to find a suitable container for the rest of the electronics if you can’t make one from scratch like comsa42 did.
Head to comsa42’s Instructables page for the full guide.