@lina PKP Intercity and regional operators are cheaper than DB, ÖBB or NS because Poland is poorer than Germany, Austria or Netherlands
ČD is roughly similar to PKP Intercity, prices are a national matter, they are not mandated by the EU, international ticket prices are mandated by the running companies but even then both operators will have its discounts, which is how tickets for Vienna-Prague trains are cheaper through ČD rather than ÖBB