Can a single live on $1200 a month in Costa Rica?

Yes! I know many foreigners who are doing it. The most important factor in living on a low budget is knowing how to do it.

Housing: Apartments can be found for around $500or a little more. Larry from South Carolina lives in a charming one-bedroom apartment in San José’s historic Barrio Amon. Everything he needs is close by: supermarkets, outdoor and indoor farmers markets, a variety of affordable stores. With kinds of products, one-public and two private hospitals that are in walking distance of Larry’s apartment. Some expats share a house or large apartment to further reduce costs while others do house sitting.
Medical care: Any person on a limited budget with a Social Security or other lifetime pension of at least $1,000.00 dollars per month, can qualify for becoming a legal resident and can join the public health care system known as, La Caja. Under this low-cost system everything is covered including pre-existing conditions.
There are NO heating or air conditioning bills in Central Valley because of year-round spring-like weather.
No automobile is needed: The country has an affordable public bus system that goes almost anywhere. I live in the city of Heredia about 10 miles from San José. A bus ride to San José or other areas in the metropolitan area usually costs around one dollar or less. A long-distance trip to any place in Costa Rica is no more than $10.00. Rates are kept very low so that everyone who does not own a car can get to work cheaply. Taxis and Uber are also much less than in the States. If one does buy a car, bear in mind that automobiles and gasoline are imported. So, are taxed heavily and are much more expensive than what you would pay at home.
Stay away from imported food products: Foods that are imported are expenses because of import duties. However, most of the local versions of similar products are much more affordable.
Bargain food: On weekends every city or good-sized town has a year-round outdoor farmers market where affordable fruit, vegetables and other items can be found at rock-bottom prices. Costa Ricans and expats flock there to get the lowest possible prices and stock up for the week. On my relocation tours we tour a huge outdoor market with about 100 stalls. My guests are amazed by the variety of fresh products and the savings.
Avoid malls and high-priced restaurants: The price of both is about as high or more expensive than back home. In fact, American fast-food chains in Costa Rica charge the same prices as in the States. To save money many expats often eat breakfast or lunch at one of the many central markets where there are small eateries called sodas. Once you live in Costa Rica you will know where to find affordable food.
Affordable clothing can be found here: Every city or town has stores called Ropa Americana where a good number of locals and expats, who are on a tight budget, can finds bins full of second-hand clothing from the United States. Believe it or not! A friend of mine bought a used two-piece suit for a wedding that cost under ten dollars. A woman lawyer I know purchased a designer purse from the U.S. for five dollars. She also buys her children’s clothes at factory outlet stores found here.
Avoid costly and wasteful habits like casinos (the house always wins) or hanging out it bars.
Internet is free in most public spaces like city parks, so you do not have pay for it.
Streaming TV programs for under $10.00 per month is available. So, expats can see all of their favorite programs and movies from home including sporting events like the NFL or NBA.
Affordable live entertainment is “free” in some case or inexpensive. Concerts and soccer games cost less than in the U.S.

Numbeo is the world’s largest cost of living database. It collects crowd-sourced data that accumulates into insights into various aspects of life in different cities and countries around the world.

“A single person’s estimated monthly costs are ₡436,978.00 or around $800.00 dollars without rent (see above).”
“The cost of living in Costa Rica is, on average, 26.4 percent lower than in United States.”
“Rent in Costa Rica is, on average, 63.0 percent lower than in United States.”

See more here: https://www.numbeo.com/cost-of-living/country_result.jsp?country=Costa+Rica