If you are on a budget, you will likely be eating at 'sodas' which serve typical Costa Rican food (mainly rice and beans). Breakfast is usually eggs (generally scrambled) with gallo pinto (rice and beans) and tortillas or bread. You will pay about $5.
Lunch and dinner can be a casado (rice, beans, vegetables and or salad, and a protein of your choice of either fish fillet, pork chop, chicken, or beef) which will run anywhere from $5-10 depending on the soda.
If you go for pizza, burgers, pasta...you will be paying about $12 and more. You will find many restaurants that will be within your budget. Many restaurants have a copy of their menu available to look at before you decide you want to eat there. You will have to just find your way around to the places in the area and see what suits you. The most expensive item on the menus are alcoholic beverages.
If you have a kitchen where you are staying, you can save some money buying fruit, beer, and beverages and what you would eat for breakfast and/or lunch and go out for dinner....If you are buying food that you find common 'at home' you will be paying a lot more that 'at home' because it is imported and costly due to importation and transportation.
For transportation from Liberia, I would suggest taking a shuttle. The cost is around $45-55. I know that there are a couple of shuttles that service the route between Liberia and Santa Teresa, you may want to try Interbus or Montezuma Expeditions to start with. Other than that, the least expensive way would be by public bus and that is an all day adventure because it stops at every small town and pueblo and goes to Puntarenas where you would have to transfer to the ferry and catch a connector when the ferry docks in Paquera.
The rains have begun early this year and you will likely get sunny mornings and possible rain in the afternoon or overnight. But, it will be warm!