If you just walk into a battery store and say "Hey, give me one of those", that price is in the ballpark, particularly if you're only buying one. If you shop around, you might be able to find a lower price, but unless it's used/blemished/whatever, I wouldn't count on getting much lower than that.
Comparing golf cart batteries to SLAs isn't always a useful thing to do, as some environments require a sealed battery, and the golf cart battery ain't.
Regaring the Optimas, they only have a little less than half of the capacity he's talking about (40-50AH, depending on model), so they SHOULD be cheaper!