I see a couple of basic questions:
1) Once you decide you DON'T want a lead acid 12v battery in your car (and I think we're seeing signs that they DON'T do well in a Leaf), your choices are limited to the AGM type batteries (that some have tried) or Lithium. If this battery lasts 3-5x LONGER than one of those, then paying 2-3x the price (which is already twice the price of a stock battery) will ultimately pay off in the long run (I didn't pay $800 for the battery). Same concept we took when we paid more for a BEV (up front) to avoid years of paying for gas.
2) You can read my blog entry on this (on MNL), but my research found that Lithium Pros know how to design batteries and the systems that control them to the point where their batteries have the most sophisticated BMS systems integrated into batteries today. As for the "cheap" 12v Lithium replacement batteries you find on eBay (and elsewhere), you get what you pay for.
Bottom line: I didn't buy this battery to save weight (although that's nice), I bought it because I expect it to last a LONG time in the Leaf.
2011 Blue Ocean SV w/OVMS
12v LiFePO4 battery w/Antigravity Battery Tracker
FIAMM 74100 horns/Wet Okole seat covers/Tor's heater mod/Dala's CAN-bridge
Lizard Pack (Rev E) installed @51 months/41k miles
40 kWh Pack (Gen2) installed @115 months/85k miles