Are those prices up to date? There are several shops in Brisbane AU that will sell drop in 12V batteris retail for 350USD per nominal kWh (about $320 per real kWh at 0.1C) including taxes. The occasional special is in the 250-300 range. Ebike batteries with higher current are in the 300-500 range.
They're highly unsuitable for an engine starter at that price though (1C BMS) and have no built in low-temperature monitoring. There are bigger systems with better safety features for about $400US/kWh available in europe and asia.
Also note that nominal capacity of a lead acid battery is often not usable capacity. I was assuming 50% DoD as usable daily capacity which may be pessimistic.
It's possible they're not! But I'm suspicious of this notion that prices have dropped by a factor of 5 since last year, and have reached Australia but not Just Catamarans in Florida; maybe they're doing the calculation differently than you are. Or maybe there's a fraud going on.
FWIW your US$320/kWh works out to US$89/MJ. (I try to use SI units when I can; it saves a lot of hassle.)
I think it's fair to exclude taxes, but not to include "the occasional special", since the retailer is presumably taking a loss in that case and will be unwilling to sell you an arbitrarily large number of batteries at that price.
A thing I wonder about is how big a Li-ion battery you really need for an engine starter. 200 amps at 12 volts is only 2.4 kilowatts; a 15C battery with 0.6 MJ capacity could do that, which is about a dozen 20700 cells. You do need a 200-amp BMS, but I think the cost of the cells is still the issue.
Are you correcting for usable capacity in your prices? 50% DoD was typical for lead acid last I checked vs 85% for LiFePO4 (or 100% if you're happy with it lasting as long as lead acid at 50%)
As to fraud, I've seen at least four batteries perform as advertised. Also a 5x disparity in prices between different shops is entirely consistent. Prices have been dropping rapidly and there are enough people who go 'oh yeah, that was about right 3 years ago'. Plus retailers may have paid several times current retail for their stock if it's a year or two old and was bought at prices that hadn't been updated for a year.
They're highly unsuitable for an engine starter at that price though (1C BMS) and have no built in low-temperature monitoring. There are bigger systems with better safety features for about $400US/kWh available in europe and asia.
Also note that nominal capacity of a lead acid battery is often not usable capacity. I was assuming 50% DoD as usable daily capacity which may be pessimistic.