[Pete] wondered how real-time clock modules could be selling on eBay for $1.50 when the main component, the Maxim DS3231 RTC/TCXO chip, cost him more like $4 apiece. Could the cheap modules ...

The RTC you ... in special modules designed for the Raspberry Pi’s particular form factor. They’re available in all kinds, from the humble DS1307 to the razor-sharp DS3231.