HS...there is more to it than that...the calculation is NOT simply cost per amp hour over the life cycle based on purchase cost...it also figures in engine run time to charge etc. which is why the spreadsheet may turn out entirely differently for your boat and you can see all the inputs on the last sheet of his site here:
Example...his fuel cost for diesel is $1.50/gal. ...closer to 3 bucks now and if you have a big engine to run it uses more fuel per charging hour. If you charge on solar/wind...it turns out differently.
It is clear to me that FLOODED are cheaper in many or most circumstances but the typical long term cruiser may
get both the benefits of AGM AND cost efficiency long term vs. flooded. Ya gotta enter the data for your own boat and battery choices to see if it works for you.
EDIT...HS...did a little research on the current Rolls/Surette marine batts and an 8d is $1600 bucks each and the model calls for two and is rated for 1500 cycles...even on your math basis that is $3200...1500 cycles and 650 amp hours.
Lifeline AGM...about $950 for 2x8d's and 1000 cycles and 510 amp hours.
That is a cost per lifetime amp hour of .00328 on the surette/rolls and .0018 cents for the Lifelines so YOUR lifecycle cost numbers with todays products from both companies favor the AGM's.
I would note that the current rolls/surette marine 8d's feature their unique 6 cell bolt together approach which is costlier to produce and has the advantage of being lighter to deal with and individually replaceable in the event of a bad cell. The warranty is also 7 years so all that is worth something as well...but on a pure "how much does it cost per amp hour over the life of the battery?" basis...the Lifelines win.