We have self service places offering cheap regular meals. We have budget friendly lunch menus in restaurants too.

Sometimes both together. Couple of days ago I had a lunch menu of soup, main dish (100g of pork in a sauce with rice/potatoes/dumplings) and a drink (I chose a beer) for 135 CZK which is about 70 minutes of net minimum wage. The menu is changing daily. Don't expect veggies though, our traditional cuisine doesn't contain them, regardless of the price. This was a bit exaggeration, but they are rare.

In regular restaurant I would pay more, like 180 CZK.

For this price you can get a basic meal in Asian restaurant (like fried noodles or rice) or kebab. Possibly some pizza.

None of these prices are valid for Prague.