The exchange rate in Costa Rica has remained a steady ₵500 colones to every US dollar for years, until recently. Over the last few months the rate has jumped up to as high as ₵580 colones for every buck! The exchange rate seems to have settled down around ₵560 for the time being. Here is where you get ripped off.
When you fork over US currency at restaurants, shops, and veggie stands they are often only giving you ₵480-500 colones to the dollar! The loss really adds up! A nice dinner might run you ₵40,000 colones, if you had them US currency and they only provide ₵500: $1 you will be overcharged $8.57 USD. The currency conversion abuse has lead to the passing of a new law in Costa Rica that requires businesses to post their current exchange rate. It is really hit or miss if businesses are in compliant.
My advice to you is pay with colones you received from an ATM, or cajero (where, typically, the best exchange rate is offered), or utilize a credit card that doesn’t charge currency transaction fees to avoid an average 10% loss.
To learn about more ways to save a buck in Costa Rica read Becoming an Expat: Costa Rica.