Subfigures placed horizontally

Don't use an empty line between the two subfigures:

\begin{figure*}[t]
\centering

\subfigure[Market revenue]{%
{\epsfig{file = figures/graphs/revenue.pdf, width = 3cm}}%
\label{fig:evaluation:revenue}%
}\qquad
\subfigure[Average price]{%
{\epsfig{file = figures/graphs/avgPrice.pdf, width = 3cm}}%
\label{fig:evaluation:avgPrice}%
}

\caption{Simulation results}
\end{figure*}

However you should be using the package subfig instead of the obsolete subfigure and graphicx with its more powerful commands instead of epsfig (which should never be used in new documents, it exists only for compatibility with older ones).

\begin{figure*}[t]
\centering

\subfloat[Market revenue]{%
  \includegraphics[width=3cm]{figures/graphs/revenue}%
  \label{fig:evaluation:revenue}%
}\qquad
\subfloat[Average price]{%
  \includegraphics[width=3cm]{figures/graphs/avgPrice}%
  \label{fig:evaluation:avgPrice}%
}

\caption{Simulation results}
\end{figure*}

Update to answer from: @egreg

1. Empty line did cause problem for me as well. Took off the empty line my images went horizontally. There are a few symbols to add space between subfigures: ~, \quad, \qquad, \hfill. (Update from @Werner: a few more symbols to control horizontal space explained here.)

2. According to this latex page on wikibooks.org, subfig is also deprecated now, and the recommended package is subcaption:

A useful extension is the subcaption package which uses subfloats within a single float. The subfigure and subfig packages are deprecated however they are useful alternatives when used in-conjunction with latex templates (i.e templates for journals from Springer and IOP, IEEETran and ACM SIG) that are not compatible with subcaption. These packages give the author the ability to have subfigures within figures, or subtables within table floats.