TD Ameritrade and Scottrade simply charge their regular commissions to trade shares in premarket sessions. Others, such as E*TRADE charge an extra $0.005 a share for pre-market trades. Brokers also often provide particular premarket policies that is usually available on their websites.
Remember that the premarket trend and price range can take a different course after the opening bell. Getting caught up in a position that backfires ferociously after the open is one of the worst situations.
There tend to be many stop orders during premarket session that are ready to trigger prices after market opens.
The orders can result in tidal waves of momentum against existing positions, which is why investors should close their positions before opening bell.
Premarket Trading Example
As you can see in the Twitter (NASDAQ: TWTR) chart above, premarket volume is very light. The only thing that can change that is news during off market hours like buyouts, mergers and most commonly earnings.
It’s very risky to trade the premarket when volume is this light because you are susceptible to illiquidity which can make it very hard to exit a position without a lot of slippage.
Above is an example of premarket trading after earnings were released. You can see there is significantly more volume (although still quite less than normal market hours) as traders are reacting to the earnings news.
Stocks can be incredibly volatile during this time BUT there is also more liquidity which will make it easier to get in and out of a trade.
If you are new to trading you should avoid trading during this time. It’s just too risky and there is plenty of opportunity during normal market hours to capitalize on.