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Income taxes changed many times during the existence of Soviet Union. In 1980s there was non-taxed minimum of 70 r per month. After that sum the tax was progressive: up to 13% for those who obtained salary from the state.
But for the small number of people whose income was from private enterprise (officially called "non-labor income") the tax was 12% to 65% depending on the income, the highest tax bracket was for incomes more than 5000 r per year. So I suppose that the lottery winner had to pay 65% tax.
My sources are in Russian:
https://www.msn.com/ru-ru/news/other/какой-подоходный-налог-был-в-ссср/ar-BBVPD5k
Remark. One could be very rich in Soviet Union even without private enterprise. For example, they payed royalties. So popular authors, writers, composers could have very large income. I suppose they were in the highest tax bracket.