Just found an interesting bit of advice from the FrugalTrader. S/He tries to keep the commission amount to about 1% of the trade amount. If the cost of the trade is above 1%, you should buy more shares!

That’s about $1,000 for every $10 of trading fees. My commission relative to the transaction amount has varied from 4.78% to 0.40%. Lately I’ve been trading larger amounts and yes, fees don’t seem to matter as much now. I mean they still do, of course, but what’s $10-15 when you’re investing $6,000? It matters much more if you’re buying just $600 worth of shares.