Polish Property Prices
There are no official sources that gather together sold property prices in the same way that Land Registry does in the UK but there are some services that aggregate together offers for sale and then store this to provide historical data.
This information is a useful guide to understand if property as a whole, or any particular area, is increasing or decreasing but bear in mind it will not tell you if prices being agreed are generally above or below asking prices. This knowledge can only be gleamed by speaking to a number of estate agents to find a consensus.
For current and historical asking prices the most accurate source is via the link below at money.pl. To use this open the link in Chrome, wait for the page to finish loading and then right hand click on any white space on the page. From the menu that opens choose 'Translate to English'.
The above aggregates data from all the most popular property portals so although these individual portals also provide their own statistics they will never be as accurate as money.pl.