I was thinking of getting a GSM shield for making a home security system with arduino. GSM modules (not shields) seem to be avaialbe for around 26$ in India (http://www.ebay.in/itm/SIM900-GSM-Modem-with-Antenna-and-RS232-interface-for-GSM-SMS-GPRS-MCU-projects-/281061937766?pt=LH_DefaultDomain_203&hash=item417098d266). But they are too big.
I finally found a shield in India for 50$.
The schematic is available here http://entesla.com/index.php?route=product/download/download&download_id=85
I want to know what features it is missing. I noticed that it was missing a backup battery option for RTC which is a really big con.