vMaD is an NGO Charity in Siem Reap offering cheap and short-term volunteer opportunities. Their minimum placement is only one week, and they were the cheapest I could find.
They pay for all your accommodation and transport and the profits go to charity. There are projects available teaching in orphanages or doing building and farming work. Probably worth checking out if this sort of thing appeals.
Updates
New website at www.volunteer-cambodia.com (the old link should redirect you there though). They also have a new program called Bakong Discovery.
Is this your business?