What does the cable have to do with it? The HDMI adapter even says "does not come with cable". If you are talking about the connector, they are $0.43 each on digikey.
If the rest of the HDMI adapter board looks anything like the VGA adapter, with just passives and a connector, then they would be making a profit charging only $3 for it, let alone the $15 they are asking.
Based on the BOM at https://github.com/NextThingCo/DIP-HDMI-PCB/blob/master/v1.0..., you've got a Chrontel CH7035B HDMI interface chip, a serial EEPROM (probably used for EDID/HDCP key storage), and a LDO power supply for those chips. That definitely counts for the additional cost.
If the rest of the HDMI adapter board looks anything like the VGA adapter, with just passives and a connector, then they would be making a profit charging only $3 for it, let alone the $15 they are asking.