Commissions.

Electronic trading and competition have brought about a sizeable reduction in the bid-offer spread (the equivalent of commissions). The spread covers the risk of the market maker. The spread for the majors remain very low, but can increase as the liquidity of a specific currency drops. Despite recent reductions of commissions through online stock brokers, the Forex market is considered, by some, to have the lowest commissions relative to trade size when compared to other financial markets. This is also in part due to the 100:1 leverage offered by most trading houses. A client with a $10,000 deposit can leverage this to $1,000,000. Some electronic communication network brokerages have introduced a per trade commision alongside a narrow pip spread.
Many retail trading houses would suggest that the large size of the market makes it impossible for a speculator to affect the market. This is not quite the truth - the stakes are higher, larger quantities of money are involved, and the bigger banks spend a lot of time and effort trying to manipulate the market. Governments have been known to step in and affect prices