This is a difficult text. It is saying that corn of similar quality, whether it is from a poor or a rich country will be about the same price at the market. The reasoning is that even though the richer countries have better systems of agricultre, they are not that much better. My guess here, as this is not in the text, is the author is suggesting one of two things: 1) The richer countries do not have agricultural systems so superior that they produce a higher quality corn than the poor ones, therfore they have to sell it for about the same price since the quality is equal. 2) The richer countries do not have agricultural systems so superior that they produce a greater quantity of corn, creating a surplus which might allow them to sell it at a price less than the poor countries.

From my reading of that page, the author is saying that of all marketable commodities, agriculture is where the differences between nations has the smallest (but non zero) effect. To put it in words: the corn of a rich country is not that much better than the corn of a poor country, unlike the roads, cars, machines, buildings, institutions, military, science, etc.