It is illegal for a foreigner to work in Costa Rica without a Work Visa. You are on a tourist Visa, so you can not work.
There are under the table jobs out there I guess if you look hard enough, but you will be not be the first person with this idea, and the odds are slim. Also, the wages are maybe $12 day. They don't tip in CR. The tourist areas the migracion police do sweeps of the bars and business with gringos working and if you don't have a proper Visa you are departed at once and black listed for 10 years.
If you can't afford to come to Costa Rica as a tourist, don't visit. Its not cheap there either, food and housing in a touristy area will be similar prices to USA. Its a been a popular destination for 4 decades, 2 million tourist a year visit CR the last decade alone.
Lastly, don't take a job from a local, thats not cool. You come from a educated western power, and are in a small developing country, think about it. Do volunteer work and give back, to those that don't have nearly the life that you come from back home.
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