Some competition:
7Ah sealed lead-acid 12V battery
12V Rechargable Spotlight which I've seen at Ace for ~$10-15, from which I pulled a 7Ah 12V battery for my bicycle's illumination system.
I'd love to know where I can buy a dozen of those batteries at a good price for an electric lawn mower project.
14.4V Power tool batteries: $160/KWh
$15 7AH 12.8V Pb-acid: $167/KWh
Wal-Mart deep-cycle marine battery (obviously unsuitable for a bike): $42/KWh