So, you want to try out authentic Khmer food without paying a $15 price tag? It's easy - just venture off the normal tourist path and have a seat with the locals.
It's easy to find a place to eat with the locals when you walk around town a bit. The best restaurants will be crowded with people. There are many kinds of food on offer near the bazaar. First, pick the dishes you want (since you don't speak Khmer then just point, they'll understand). There is always rice, some fish soups and meat dishes. I recommend picking the fullest pot because it won't have been sitting out long. Then, take your food and take a seat at a large table, elbow to elbow with Cambodians. There are spoons, forks, and sometimes chopsticks in the middle of the table.
Next, try and eat all of your food without making too many faces. Since they probably won't speak enough English for you to ask the ingredients, try to prepare your taste buds for some new, unique tastes. Vegetarians, the only thing that is definitely meat-free is the rice, unfortunately.
When you're all done you can pay (or you may want to pay beforehand in order to avoid any confusion). For rice and another dish it will probably cost less than 1,500 riel (40 cents). What a deal! The staff with clear your dishes.
For light breakfast, there's some bakeries scattered around the city. There's often a large bin of fresh baguettes (delicious!) and different treats available - a legacy of the French. This food is also cheap, as a danish may cost 1,000 riel and a baguette around 500. So go out there and find the authentic places and dig in!