How much does it cost to buy a book copyright?

I think that your question would be better addressed to an attorney. Since your country is not a signatory to the international treaty, it may be more involved than simply “obtaining the copyright” since your country won’t recognize its import or value. On that basis, the copyright holder may not even be willing to license to you. (That’s another thing: I don’t think that you “buy” a copyright to another’s work; I think you’ll “license” it. But I really don’t know. I have no special knowledge in this area.)

So while it may be possible for someone to give you an approximation of cost, that’s like asking “how much will it cost to have my kidney transferred to my child?” Cost is one of the smaller pieces of “an awareness of the entire process”.

In addition to @CWOTUS‘s guidance, one must also consider the sales value of a translation. A translation of my poetry would not sell a single copy; a translation of The Hand Maid’s Tale would sell many many copies.