Bus is by far the cheapest option to travel between Phnom Penh and Siem Reap, costing from US$6.
Bus
There are dozens of companies operating between Phnom Penh and Siem
Reap. The journey takes 6 hours and passes through some nice
scenery. Buses leave throughout the day giving you the option of
a late or early start. GST Express are one of the cheapest
companies, operating comfortable buses with AC, on board movies and
allocated seat numbers. Their buses leave their depot by Central
Market, but guesthouses selling tickets should provided a free shuttle
to the market. Other companies offer more expensive tickets
(about US$9) for more modern buses with on board toilet and provided
water.