To expand on this: one can operate as a sole trader company, and enjoy the 19% flat income tax rate. You can also register your company for VAT, and then claim back the VAT on company purchases.
The only other cost is ~$270 / ~240 EUR / ~180 GBP per month in social insurance taxes.
I've been here for the last five years, and the low cost of living (very low cost outside of Warsaw) combined with low taxes makes Poland a very forgiving place to be when your first few side businesses inevitably fail.
(for that matter, the startup community here in Warsaw, centred somewhat around Reaktor - http://reaktorwarsaw.com - is really picking up steam)
If you do what I do - remote consulting for clients in the UK and USA - and you have the talent to charge western rates, you can build a sizeable nest egg in next to no time.