I just got back from Siem Reap today! We had a great time.
We paid about US $55 per day for an airconditioned van and English-speaking guide -- this was for the whole group, so split amongst the nine of us it was very reasonable.
We booked our hotels and flights via a Free & Easy package from Tradewinds - came to about S$350 per person via SilkAir, staying at Casa Angkor hotel. It was a 3-star hotel, pretty decent. Although, I read in the SilkAir inflight magazine (on the way back!) that Casa Angkor is offering promotional rates of US$25 a night until September 07... might have been cheaper for us to book directly.
There are lots of guesthouses and B&Bs in the Old Market area, which is where all the good food is
Meals were about US$3-5 per dish, which can usually be shared between two people.
Enjoy your trip!