Motel One offers a discounted price of 328 PLN per night, including breakfast

https://www.motel-one.com/en/hotels/warsaw/hotel-warsaw-chopin

Make your reservation by email  res1.warchop@motel-one.com.

Please cite the code "Relevance" and my email address a.piskorska@uw.edu.pl   in the content of the email. 

Other hotels located near the conference venue:

https://www.barcelo.com/en-ie/barcelo-warsaw-powisle

https://www.savaboutiquehotel.pl/en

There are also plenty of apartments available via booking websites