You're resistance to go overland put a big crimp in the budget aspect of going to Siem Reap.

I would recommend, on a budget, that you fly to PP on AirAsia (about $100us R/T). From there take the boat, or a taxi, to Siem Reap. Boat is about $20, taxi is about $30. See Siem Reap, and Ankhor Wat, then return to PP by taxi, or bus. Total cost will be less than just the flight from BKK to Siem Reap. Bangkok Air has the monopoly on that route.
The road from PP to Siem Reap is good, but if you want to fly the whole thing, then forget your budget. The flight from Siem Reap to PP, or vise versa is $157, again on BangkokAir.
If you are insistant about flying, then the cheapest is about $260us just for the flights.
If you relent, and go overland from PP to Siem Reap, the whole trip including hotels will cost you about the same as just the flights.

Just a thought.

koolbreez