Poland is a relatively inexpensive country, especially for tourists coming from the West. Cities are certainly more expensive than rural areas, with Warsaw, Krakow, Gdansk and Wroclaw being the most expensive ones.
Sample prices of selected goods and services in Poland:

Goods and services PLN
Petrol (95), 1 litre 5,70
Double room in a ***hotel 350
Taxi, 1 km 2,20
Bus ticket, single fare 4
Lunch in a popular restaurant 25
Three-dish dinner in a popular restaurant 50-70
Pizza in a popular fast food restaurant 25
Coffee in a bar 5-10
Beer in a pub, 0,5 litre 10
National museum ticket 20
Cinema ticket 20
Theatre ticket 50-120
Train ticket Warszawa – Krakow, Intercity 1st class 135
Mineral water, 1,5 litre 2,5
Milk, 1 litre 2,5-3