The cost of living in Cambodia really depends on how you like to travel: either like a Khmer, eating in local restaurants, or in a more European way. In any case, you should know that the tourist excursions are often quite expensive, and they don't reflect day-to-day life in the slightest.
Throughout the country you will find excellent local restaurants offering meals for a maximum of two dollars. The same can be said for the towns that really get going at night, most notably Siem Reap and Phnom Penh - the prices in the bars there are very different to the UK! It's usually one dollar for a beer, and two or three dollars for a cocktail...
There are also lots of free excursions, like to a lotus farm, the Angkor artisans or the rice fields! You can easily spend time in Cambodia without spending lots of money.
Siem Reap is a guesthouse paradise, and you will find a lot of them in the city centre, most notably on Sivatha Boulevard, with rooms starting at 8 dollars per night, sometimes with a swimming pool.
In terms of visiting, the official monuments and museums are not free - the city of Siem Reap itself is not that cheap! The national museum costs 12 dollars (about £8) and a day in the temples of Angkor is available for 20 dollars (around £13). This is not unusual, given that this is the place that attracts the most visitors, and is a 'must-see' during a visit to Cambodia.
As far as hotels are concerned, there are guesthouseseverywhere, but also there are very beautiful hotels at more reasonable prices than Europe. At Siem Reap, you can find the Sofitel and Park Hyatt chains, which offer international quality at good prices - ideal if you want to treat yourself! The independent hotels in the area offer good quality for less than £28 per night.
Obviously there are those were you can really spend your money, for example the Amansara hotel in Siem Reap and the Song Saa in Sihanoukville, where you can have a private meal in one of the temples.