You can use otodom, it has resources at the bottom of individual listings showing average prices of offers on a city by city or region by region basis. Its not perfect but its free. Below is a link to ceny szybko, also quite useful for comparing rents/prices.
ceny.szybko.pl/Wroc%C5%82aw-ceny-domow.html
The information is only as accurate as what is put into the system, so take it with a pinch of salt and look at local offers and how long they have been listed for to gain an idea of true prices.