Some 100AH AGMs are 27 size footprint so replacing six 6s might be an issue for tray size. BUT--AGMs can go inside as long as you don't overcharge them so they pop their gas valves. So you can escape the tray. This gives you options as to placement and battery bank AH size.
Note AGMs inside where it is heated, do not lose capacity in colder temps, which has the effect of more capacity than you would be used to in winter.
Can't help with pricing per AH but just tossing in the additional placement factor