I found this super light (4 gram) mini OSD on the Hobbyking site. Price is only US$ 8.95 and maybe lower if your are long time member of Hobbyking.
Super Simple Mini OSD
Easy to install and configure the Super Simple Mini OSD connects directly between your cameras video out and the transmitters video in. 2 Super Simple Mini OSD units can be used together to display voltage for up to 4 individual 2~3 Cell batteries.
Voltmeter & Stopwatch
Double Voltage Input (Up to 4 voltage inputs, requires 2 x Mini OSD units)
Programmable Display Content
Supports NTSC & PAL
Dimensions: 51 x 18 x 8mm
Working Voltage: 7.2~12v
More infomation at the hobbyking site: http://www.hobbyking.com/hobbyking/store/uh_viewitem.asp?idproduct=19369&aff=132929