I had the 55 Ah batteries on my mobility scooter replaced yesterday. There are two battery replacement stores in the suburb, neither had that size of battery available although one did say if I wanted greater range I could upgrade to 75 Ah batteries that would fit. I enquired about price and was told $505 each. The alternative supplier didn't have any of the 75s in stock but could get them in for $399 each.
I mentioned this cheaper price at the first store (the one that had then in stock) and he matched the lower price of his competition. I knew they'd be pricy, they are a sealed, deep cycle gel-electrolyte lead-acid traction battery, vastly different to the regular vehicle flooded lead-acid unit.