Wow, the batteries for this bike are cheap. I wonder if that is the cost of the entire pack or just one of the cells in the battery this bike uses...
http://www.electricmotorsport.com/store/ems_ev_parts_batteries_thunder_sky.php$220.
Thunder Sky is one of the largest manufacturer of lithium batteries in the world. The chemistry of these cells is Lithium Iron Phosphate (LiFePO4) or LFP. The main advantage of LFP batteries is that they do not have the volatile thermal issues that Li-ion batteries have, making LFP batteries much safer then Li-ion. The other advantage is that they have very high cycle life of up to 2000 cycles compared with Sealed Lead Acid (SLA) batteries that have a cycle life of about 400 cycles. This means that the LFP cells can last 5 times as long as the SLA batteries. The price of lithium batteries keeps coming down and the price of lead batteries keeps going up. Hopefully lithium batteries will eventually become equal in price or even cheaper then SLA batteries. If you factor in the cycle life of LFP batteries it can be argued that LFP batteries are already cheaper per use then SLA batteries.