I don't know. If you must buy souvenir you should buy it there where you can, but always the prize must be acceptable!
Unique Suggestions: Don' buy souvenirs around the most popular places in the city. There is the most expenisive souvenirs!
Fun Alternatives: You have a lot alternative! You can buy cheap, but propably "trash" souvenir on the bazaars( but always souvenir ;) The best place to buy cheap and good or a bit worse souvenir ( please correct me if I make any mistake :) is STADION DZIESIÊCIOLECIA. There you can buy everything! Really!
If you're going to visit a bunch of cities in Poland, then do not buy any souvenirs in Warsaw. The selection is poor, and the price is much, much higher than anywhere else in Poland. The best places to go are Gdansk and Krakow for Amber jewerly, crystal and glass, and Zakopane and Krakow for local crafts such as wooden boxes, leather crafts, and wool and linen products. Zakopane has by far the most selection of these local crafts, and they might be as little as a quarter of the price you would pay in Warsaw. As a general rule, everything in Warsaw will be much more expensive than anywhere else in the country, and often, Poles refer to anything overpriced as "Warsaw prices." Here I am with Sylwia in Old Town trying not to blow the rest of our money away.
When you stroll along Nowy Swiat St. to the center, that 'pedestrians only' way, remember that all the shops there are very "tourist oriented" and priced accordingly.
Buy yourself a small bucket of raspberries (about 1 USD) or cherries (depends on season) and walk around avoiding those over priced shops.
The biggest tourist traps are hotels. NEVER, EVER buy there. Their markup seems massive.
Unique Suggestions: Buy basic souvenirs like fridge magnets, post cards, posters, books, tshirts, etc.
Fun Alternatives: I found the museums, galleries and a few mom and pop's stores to be great alternatives where you can get unique souvenirs for a fair price:
1. Muzeum Narodowe,
2. Zamek Krolewski,
3. JMK on Krakowskie Przedmiescie
4. Palac Kultury
They carry items made by citiline.com.pl which has got some innovative items at a decent price.
Just my two cents.