No, Heyah isn't the cheapest one.
I think Orange Free na karte will be the best.
After recharging you get many GB bonus.
For example when you recharge 50zl, you get 2,75GB at all. You have 50zl credit, which you can use for data at 0,01zl/100kB rate or calling at 0,29zl/min + 2,27GB data bonus. It's valid for 90 days.
Other recharges you have here:
http://www.orange.pl/binaries/PL/Bin...60/4669587.png.
You can buy any Orange prepaid sim anywhere and change your tariff (before recharging!) using *101*40# code.
There is also GaduAir - there are some packets - 300MB/11zl, 100MB/5zl - both valid for month.
Other option is Play Online, but it has very limited coverage. 30zl recharge - 2GB/month, 50zl - 4GB/2 months. No calling, only data.