Very small 12V DC power supply
A full iPhone charger teardown is given by Ken Shirriff. Making something that small that is efficient (doesnt produce much heat) while also meeting the EMC and safety constraints is a very difficult task, and is only cheap because of the extremely high volumes that companies like Apple can produce.

Apart from the size of the major components such as transformer and transistors, one of the factors that stops these chargers getting much smaller is the mandatory spacings for safety agency approval. Pads/pins/tracks with high voltages between them have to comply with creepage and clearance distances so that they don't electrocute the users.

Since your priority is for a small size PSU, you have to compromise on the cost. The absolute cheapest power supply may not be the smallest, and as Ken shows, may compromise safety or performance. The Apple design seems to cost about 5 USD for the parts, which is cheap for such a high quality design. Their selling price, as with much consumer electronics, may not directly reflect the cost of the parts.