I had an old Discovery roomba that been dead for a whie because the battery been dead.
At 36 bucks he''s back to life.
Seems to be cheaper than you can find at other sites.

All-Battery.com has a Replacement Battery with hard case (Yellow Color) for iRobot Roomba APS 4905 400 series Vacumm Cleaner for $36.66
Free Shipping with code tenergy2008
Link
http://www.All-Battery.com/replacementbatteryforirobot400series.aspx
TECHNICAL SPECIFICATIONS:
Votlage: 14.4V
Capacity: 3.3Ah
Max. Discharging rate : 10 C at24 A
Dimension: 7 x 2.2 x 2.3 inches ;
Weight:38.5 Oz.

Seems to be good at 3.3Ah... and 100 minutes run time.