I own a bunch of USB to serial debug board based on CH340G/T or PL2303HX, and all include a crystal on the board with the exception of Hardkernel’s USB-UART module kit based on CP2104 (QFN24 package) working without crystal, hence smaller than most, but a bit more expensive than most.
WCH has just released a new form version of their CH340 that does not require a crystal, comes in a tiny (3x3mm) MSOP10 package, and costs about 42 cents on Electrodragon (5 pieces). You’ll also find it for as low as 1 CNY ($0.15) on 1688.com.
CH340E is slightly more expensive than CH340G, but considering you don’t need a crystal, and it will take less space on your PCB, total BoM cost for this USB to TTL function should be lower. Electrodragon reports that only two external parts are required, and they tested it up to 150,000 baud to flash ESP8266. CH340E is software compatible with CH340G, so the same drivers will work, and most of the electrical characteristics are the same, as you can find out in the product page.
The board above just measures 16×16 mm, and sells for $0.99 + shipping. In my case, shipping is $1.47, but I’d except that to soon go away, as some other CH340E boards go for just $1.01 shipped. Some CH340G boards are even cheaper, but the main point is that you can now add USB to serial to your design with minimal cost and area.
Thanks to Zoobab for the tip.