Siem Reap itself is cheap — beds, food and beer are among the cheapest in Southeast Asia — and then one fixed cost dominates everything: the Angkor pass.
What a day costs
Food and beds are the cheap part; market plates run a dollar or two. It is the published fixed costs that add up: US$7 for a foreign adult at the Cambodia Landmine Museum, US$18–38 for a Phare circus seat, and the Angkor pass sitting on top of all of it.
Buy the Angkor pass from Angkor Enterprise only
The government's ticket office on the road out of town states on its own site that it is the only official seller, issuing one-, three- and seven-day passes covering ninety-odd temples. Anyone in town offering a cheap pass is selling you a problem at the checkpoint.
Cycle the small circuit
Angkor Wat is about six flat kilometres from town, and the small circuit past Angkor Thom, the Bayon and Ta Prohm is a comfortable day on a bicycle if you start before sunrise. Bikes hire cheaply all over the old quarter; you still need the pass.
Buses, booked direct
Giant Ibis runs Siem Reap–Phnom Penh on the expressway and an overnight sleeper down to Sihanoukville for the islands, and its own site undercuts the guesthouse desk.
Two short temple trips, not one long one
The park is huge and the middle of the day is unusable for much of the year, so going out at dawn and again at four gets you more per dollar of pass than eleven hours in the heat. Decide that first, then choose the pass length to match.