Edward Mallon @ edwardmallon.wordpress.com gives us a detailed explanation about cheap RTC modules found on ebay and other places.
I built the first few beta units with the DS3231 Chronodot from Macetech (about $18 each), but I kept on stumbling across cheap RTC modules on eBay, Amazon, etc. and I eventually bought a couple to try them out. While I waited for them to ship on the proverbial slow boat, I did some digging, because these modules were (almost) selling for less than the chip itself if I bought them directly from trusted sources like Digikey, Mouser, etc.
Using a $2 DS3231 RTC & AT24C32 EEprom from eBay – [Link]