Here's my very recent ESXi build, all for $300 before tax. Has dual onboard Intel Gbe NICs and all!
Jetway NF594-Q170 mobo, $60, brand new: https://www.ebay.com/itm/Jetway-NF594-Q170-Intel-Core-LGA1151-Mini-ITX-Motherboard-with-iAMT-11-0-vPro/283678692085?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
i5-6500T cpu, $80, pre-owned: https://www.ebay.com/itm/Intel-Core-i5-6500T-2-50GHZ-Processor-Socket-1151-Quad-Core-SR2L8/193402037388?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649
mini-itx case w\ psu, $78, brand new: https://www.ebay.com/itm/Mini-ITX-Case-3-0-Aluminum-Computer-Case-HTPC-for-Mini-ITX-PSU-Power-Supply/312330736269?ssPageName=STRK%3AMEBIDX%3AIT&var=610969332054&_trksid=p2057872.m2749.l2649
2x4 GB ram, $39, brand new: https://www.amazon.com/gp/product/B00NLTGW0Y/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
128 GB mSATA ssd, $30, brand new: https://www.amazon.com/gp/product/B07GZLJD5R/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
low-profile cpu cooler, $15, brand new: https://www.amazon.com/gp/product/B073XSRJ82/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
Enjoy!