I thought so also, since I'm a poor student with hardly any salary. But I went to Phnom Penh and Siem Reap on a really tight budget! Cos I went by road.
Airfare SG to Bangkok - $130 (jetstar)
Bus to thai/cambo border - <$10
Taxi from cambo border into phnom penh - $30 (only cos we booked the whole back seat so it was $30 each)
Phnom Penh to Siem Reap - US$4 (bus)
Siem Reap to Thai border - US$4
Thai border to bangkok - S$4
Not so bad right?
Can read
www.talesofasia.com for more on how to do that. =) And it's not as unsafe as people think it is. I went with another friend, just two of us girls, 20 years old. No problem at all. Staying at a decent place in Cambodia is only like US$14 a night, and quite luxuriously already - with air con and toilet and decent neighbourhood.