Let me tell you where one can eat a good lunch, very cheap in santiago.
If you are in Downtown santiago try the freshly cooked meal in "el Encanto" (2nd floor). The food actually has flavor and you get a choice of rice (white or flavored) a choice of meat (Beef, chicken, pork, pork chops, even sea food) and a choice of salad (3 different salads) all for about 50 pesos.
Supermercado Salvador has the exact same setup and the food is delicious (bartalome Colon Av).
Both places above are clean with Full A/C.
If you are Criss colon (500lbs) then I suggest eating at "Mr. Cook". They have a delicious lunch buffet which includes a delicious soup, a variety of salad and delicious rice, meat variety and italian pasta. All for 120 pesos and you can eat untill you can have yourself carried to your car by the wait staff. All you can eat.
This place is located right near Plaza Internacional.
If you are at the monument you can eat in a corner restaurant (francia with Calle del sol) called "Mana". They have delicious food plates for under 50 pesos. Open air, no a/c.
Don't eat in Supermarket Nacional and Super Pola. They have a cheap Plate of the day (worthless) but if you would like to eat their other dishes be prepared to pay by the pound. They weigh every tiny little piece of meat and tomatoe. I once bought lunch there and it cost me 100 pesos (in the old days when peso had some value). These places are all rip offs.
About Ahi-bar:
Order the Ahi-Bar fillet mignon (comes with creamy mushroom sauce) or simply order charrasco Ahi-Bar (its a delicious tender strip of steak). Both dishes are around 195 pesos each and comes with a choice of french Fries, mashed potatoes, tostones and steamed veggies. I like to eat it with mashed Potatoes.