the financial costs for an effective nuclear deterrent is too high
It is prohibitively expensive to build a nuclear triad. You need to build submarines, strategic bombers, and intercontinental ballistic missiles.
However, Poland doesn't need any of that. Instead, Poland should develop primitive dirty bombs, strategically dispersed around the country.
This way, in true Christ-like fashion, Poland will be able to fully irradiate its land in the event of a foreign invasion - denying the land to the aggressor.
This unique, "I'm gonna kill myself if you take one step closer" approach, perfectly matches the spirit and history of Poland.
It will signal to the rest of the world, who the true moral leader is. Other countries build nukes to kill others, while Poland is the only country that developed nukes to use against itself. Shocked by such unparalleled selflessness, the international community will not enforce the provisions of the NPT.