When we were there last year, a lot of restaurants were way more expensive to eat in than restaurants in Canada, and the conversion rate was 2 to 1 everywhere, so if you carried U.S. money, you lost more than the actual exchange rate.
If you change your money, then you'll obviously get a better rate.
When I said "holding US dollars," I didn't mean literally holding US dollars. I meant Costa Rica is generally cheaper than the United States for people whose primary income, or wealth is in US dollars.