I second what wvuwhat posted.
In the UK commission is £15 per trade - so £30 over all once you have purchased and sold the stocks, as well as the stamp duty.
If you buy £20 worth of shares, once you factor in the £30 commission, plus stamp duty, you need to make over £30 profit just to become even.
I don't buy anything less than £2000 these days to make a good return.