If you wanted to send a letter to Spain today, you would pay $1.10 for the privilege. For that matter, if you wanted to send a letter to Hong Kong, or Australia, it would cost the same thing.
To put this in perspective, here is an item that was mailed to Spain in 1864 from the United States. At that time, there was no postal treaty between Spain and the United States, so there was no way to pay for the entire service of delivering a letter between the two nations. In other words, you would pay some of the bill and the recipient would pay some of the bill in order to get the mail to its destination.
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In this case, the sender paid 24 cents - of which 21 cents actually went toward the services rendered. The recipient paid 8 Reales (40 cents equivalent) for the honor of collecting the piece of mail. It took 61 cents in 1864 to send and receive a letter from the US to Spain. Consider this - 24 cents in 1913 is the modern day equivalent to $5 and some odd cents.