user: Question: What was the transfer fee when Winter was the transfer window?
Columns: ['Nat.', 'Name', 'Moving to', 'Type', 'Transfer window', 'Transfer fee', 'Source']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['CRC', 'Gómez', 'Deportivo Saprissa', 'Loan return', 'Summer', '—', '—']
SQL:

assistant: SELECT "Transfer fee" FROM "Table" WHERE "Transfer window" = 'Winter';