The author concludes that farming using irrigated seawater would be cheaper than other kinds of irrigated farming if it’s done near oceans. This is because, if done near oceans, the water wouldn’t have to be pumped far, and the distance-based expense of pumping is the biggest cost involved in irrigated agriculture.

The referenced text is support for the conclusion. Because the greatest expense involved in irrigated agriculture is pumping, reduced pumping costs would make seawater farming near oceans cheaper than other kinds of irrigated farming.