By most standards, Phnom Penh is a relatively inexpensive place to live. People familiar with shopping in Phnom Penh learn where to purchase many products locally at reasonable prices.
Unless you are into buying only imported or luxury goods or shopping exclusively at such places as Aeon Mall, Lucky Department Store or Bayon Supermarket (which have arguably the highest prices in the city), you can live here for a reasonable amount per month.
Certainly, your personal needs and your lifestyle will ultimately determine how much of your salary you spend and how much you are able to save. For example, if you eat out several evenings each week at the expensive hotel restaurants, you will spend a good deal (possibly between $35 - $55 per couple for each meal with drinks).
On the other hand, eating at home or at the many excellent inexpensive restaurants around town can be much less expensive (possibly between $15-20 per couple for each meal with drinks). It is fair to say that a teaching couple at NISC will be able to save at least one of their two salaries and maybe more per year.