Phnom Penh is one of the cheapest capitals in Asia to sleep and eat in; what costs money is the day you spend at Tuol Sleng and Choeung Ek, and the transport in and out.
What a day costs
A dorm bed and market meals put a bare day well under US$20. The fixed costs are the ones with published prices: Tuol Sleng is US$5 for adults over 18, US$3 for ages 10–17 and free under 10, plus US$5 for the audio guide, cash only in riel or dollars.
Buses in and out, booked direct
Giant Ibis runs the expressway routes to Siem Reap and Sihanoukville plus an overnight sleeper, and booking on its own site avoids the guesthouse desk mark-up. It takes KHQR, cards and UPI.
Base yourself in Toul Tom Poung
Rooms around the Russian Market run cheaper than the riverfront, and the market covers breakfast and lunch without a restaurant bill. The trade is a tuk-tuk each way to the palace end of town.
Use the tuk-tuk apps
Rides ordered through a phone app are priced by the app rather than negotiated, which removes the tourist quote you get flagging one down on Sisowath Quay. Sort out a local SIM or use guesthouse wi-fi before you rely on it.
Dollars up front, riel for the change
Cambodia prices in US dollars and hands back change in riel at roughly 4,000 to the dollar, and plenty of places — Tuol Sleng among them — take cash only. Draw dollars, keep small notes, and do not expect a card to save you.