Try Asia Tasty in Hala Mirowska. You can stuff yourself for under 20PLN (main courses are about 12PLN to 14PLN including rice and salad) and the food is pretty good.
If you want to spend more like 40PLN then try Red Orange.
Here's the review I wrote a month or two ago for WiK English edition:
The quality of an ‘ethnic’ restaurant or bar is often reflected by the customers: Cork Irish pub never has any Irish customers, largely because “it’s plastic paddy”; Bradleys on the other hand is superb and packed with Gaels. Asia Tasty serves, surprisingly enough, Asian food and is one of the very few places in Warsaw where you can expect to be in the minority if you have white skin.
Pl. Zelaznej Bramy 1