Ouch! You are going to be savaged for this!
Again.

So I should buy two 100 amphr battery's and parallel them to make 200amphr and will cost me $440 rather than the $980 for the 200 and same lifespan? It's hard to get my head around that!

Maybe think along a different line. How good are the warranties on the respective batteries??

I bought two 105 amp hour AGM batteries in Feb 2010. They are still going strong. A 40 watt panel keeps them charged up. They don't get a hard life but have been run down below 50% on occasion.

I have 3 x 120Ah batteries which run Fridge, Freezer, Auto helm, Navigation equipment etc. This is not enough to get me through a night with all running so I think I will be adding at least one more 120Ah.
If the 200Ah and the 100Ah are exactly the same spec other than capacity and if space and weight is not an issue then it is a no brainer that the 100Ah are the way to go.

my two cents
let me make up unprofessional guess
price is no related to quality, capacity, nothing electrical just the quantity of sales

not true
very good to charge them parallel
even better discharged parallel
when there is No charge or discharge ....disconnect parallel

Yes there is no limitation charging batteries parallel, as long as they all have the same chemistry.
unequal capacity or the same capacity with unequal discharge ...does not matter at all

they all have the same voltage but different rate of charging,
smaller or less discharge batterys are top up faster and all available current goes larger bat.

They nicely equalise, because they both on equal voltage...there is no flow between them.
On discharge ....the same process.

Problem with losing the capacity starts, when all is switched off, but bat. are still interconnected.
..it is quite simple....isn't it.....