Ahh, your boat is quite a bit bigger - mine has a max rating of like 550 lbs, so between 2 full size adults and about 30lbs of motor, a bit of equipment, another 60lbs of battery is quite significant.

I've heard that LiFePO4 batteries have been plummeting in price over the last few years - and I saw the same $300+ batteries you did. Apparently, a few years back they were much more than that even. However, there are a handful of well-reviewed newer listings on Amazon that got posted in the last year or so that are massively undercutting those prices. Honestly, I'm unsure whether this is the market at work to put price pressure on a growing industry or if these are full on and well organized scams that Amazon just hasn't caught yet.

One $170, posted just under a year ago with quite a few well written reviews (incl trolling motor usage): https://www.amazon.com/Lithium-Phosphate-Rechargeable-Off-Grid-Applications/dp/B09Y3DNKB2

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Another $140, posted even more recently and even less expensive: https://www.amazon.com/gp/product/B0BFDQSG88