China Mobile Vice President Jian Qin: "5G mobile phones will be reduced to 1000-2000 yuan next year." (Google translation) (US$145-289) This corresponds to my earlier report from Peter Chou of Huawei that he is targeting $300 phones in 2020.
The question shouldn't be how they can go so low but rather "Why are 5G phones so expensive for now?" 5G phones are no longer mysterious. They require a better processor like a Qualcomm 855 or Huawei Kirin 980. A 5G modem and larger battery are also needed. The hardest part is the radio frequency front end (power amplifiers, transceivers, and especially filters.) 5G uses many more frequency bands. At the high speeds, only a few companies can make the parts.
The difference in the costs of parts (bill of materials) between 4G and 5G models today should be US$40-75. That will come down next year as MediaTek and UNISOC ship the 5G chips they have announced. Four companies - Skyworks, Qorvo, Broadcom/Avago, and Qualcomm/TDK - offer the RF parts, with several Chinese companies racing to deliver similar products.
Decent 4G phones this year cost US$200-300. Even US$110 buys a usable phone.
Next year, as always, costs will come down.