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"The 'old' lead acid costs around 1/3-1/4 of the gels, lasts perhaps 1/2 to 1/3 of the time of gels, needs no super chargers, may be bought almost everywhere."
Sounds like you got some bad numbers there. With the exception of the Optima brand, AGM's are nowhere near that expensive. Optima carries about a 25-33% premium over other AGM batteries and a lot of people think they are the only AGM battery source. That's a myth.
The premium you will pay for flat plate AGM's, compared to flat-plate wet cells from a brand name, is about 20-25%. Or was when I checked last year. That's all.
Now, can you buy wet lead cells cheaper? SURE. But in some areas, Costco and WalMart don't sell deep cycle batteries at all, so that's really pushing the comparison to troll for bottom prices in markets everyone can't get to. To be fair, you need to compare common markets and brands.
And even then, the folks who build those batteries will be the first to tell you they are custom OEM products which may not be the same as their brand-name products coming out of the same plant. If one plant makes 20 brands--they aren't all made the same.
This is also ignoring the "real serious" batteries, the industrial grade deep cycle batteries (made as AGM or wet) and not seen in the consumer market either. The Trojan "golf cart" batteries cross into this territory, you won't find them in retail stores. The folks who do sell them, probably also sell industrial cells.
What you get for the 25% premium is a battery that doesn't leak, and can't be topped up. That also means if they are submerged in salt water, they will not release toxic chlorine gas, and they will still be functional when they come back up. AGMs accept charging 25% faster, so you can burn less fuel and make less noise. And it supplies more power to high loads. Long term performance? Ah, that's being debated and the real answer is probably "it depends on construction" so there's no way to be sure yet, aside from warranty and reputation. And, it may need some adjustments to the charging system to be charged properly.
Prices? Yah, that's going to vary, regionally too. But *overall* only about 25%. What gets harder is finding a local course of AGMs, unless you go to the Optima brand and pay for their advertising budget. And, you'll get 5-10% less power in each cell, because of the spiral technology. It's supposed to be a robust, rugged, cost-efficient alternative to flat plates, and it probably is. But it puts round pegs in square holes, so it wastes capacity, and the patent holder apparently is extracting a stiff premium for it.