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The ticket from Chop to ZΓ‘hony (for the border crossing) can't be bought online, but you can buy one at the train station in Chop. The last price information I could find is a few years old, but it used to cost about UAH 39 (appr USD 1.50).
The ticket for your connection from ZΓ‘hony to Debrecen is HUF 2,350 or about USD 8.50. You can buy it up to two months in advance from MΓV's web page.