According to Polish law, a foreigner entering the territory of the Republic of Poland must have means of subsistence of at least:
• With a return ticket: minimum amount per day of stay (approximately 200 PLN/day)
• Without a return ticket: cost of return ticket + minimum amount for each day of stay
Or an equivalent amount in foreign currency.
Proof of sufficient funds may include: cash, bank statements, credit cards, sponsorship letters.
Source: https://www.gov.pl/web/mswia-en/entry-and-residence-conditions-for-foreign-nationals-in-poland