And 50% (or better) the price. I've seen the Trojans go for $400+ each, though $300 is more common.
For long term boondocking total capacity is a marginal issue. The genny picks up after the first draining, 3-5 days. Then it is just how long vs how often the genny runs.
At $75 each another battery will give you even greater capacity and still cost less. So as I first said, it comes down to your goals. Do you want to spend the cash now for battery life in 5+ years that you might not ever use?