(FORTUNE Magazine) – If you haven't shopped for an online broker lately, you may not have noticed that fierce competition has driven stock-trading commissions to surprisingly cheap levels. Upstarts such as Just2Trade, SogoInvest.com, and TradeKing.com now boast commissions ranging from $2.50 to $4.95. Financial giants Wells Fargo and Bank of America have gone even further, offering a large number of free trades to customers with minimum balances of $25,000. The lowest of the low, though, may well be upstart Zecco.com. Launched last October, Zecco gives up to 40 free stock trades a month to customers with a minimum investment balance of just $2,500. Zecco says it will make money from interest income and margin spreads, as well as by charging a low fee for options trades. Zecco also hopes to attract advertisers to its financial portal site.

As prices go, you can't beat free--but trading costs are far from the only consideration when choosing a broker. Speedy and accurate trade execution, research tools, margin-lending policies, and solid customer service can outweigh the value of cheap trades. And getting a better interest rate on uninvested cash can more than make up for any commission savings. (Many default sweep accounts yield 1% or less, though much higher-yielding money market accounts are usually available.) What's more, if free commissions tempt you to trade more often, your returns could suffer. Zecco.com, for example, bills itself not just as a brokerage but as a financial community, and its site features a number of bloggers and discussion forums. One bad trade based on a hot tip from a blog post could more than wipe out any benefit of those appealing commissions. Just remember: Even a free trade can turn out to be expensive.