You can buy a VCC with abt 1-5$ on it, you can also make EDU emails fairly easily, make a .edu email, register for amazon, start amazon prime free trial and link VCC with 1-5$ on it and boom, you have a month of Amazon prime for 1-5$
Not sure how the people do it though where they can sell them for lower and still make a profit other than hacking them though
(the vcc needs some money on it because companies typically make a small charge to verify that it's a real card and then refund the money once it goes through)