If flying to REP is too expensive, buy a cheap ticket to BKK (or take the night bus). Take a bus or hire a taxi (bargin to approx 2000 BATH) to the Cambodian border. Get your visa and cross the border and then hyre/share a car or take the bus to Siam Reap. If you leave BKK in the morning you should reach S R by evening. Be prepared for an adventurous trip!

You can get more on this trip on talesofasia.com