hi Miriam, thanks for the great information. I just wanted to answer a couple of things you weren't sure about:
Prices in better restaurants are priced in US dollars (don't know about fastfood)
Fastfood restaurants are in colones. In San José also most restaurants outside of the hotels have the menu in colones. You will find it easier to pay in colones, but as long as you know the exchange rate you will find that the restaurants are very fair about accepting dollars.
Lots of places do NOT want $50.00's or $100.00s, why? I don't know.
This is for fear of accepting a counterfeit bill. Also the banks used to be much more strict about US bills that were slightly torn or marked in any way. Now they are more lenient, but many businesses are probably still leery of large bills for this reason. The bill may be legitimate, but if the bank for some reason doesn't want to accept it they can afford a $20 loss more than a $50 or $100.
Only bargains are golf courses.
In my opinion, dining out in Costa Rica is a great bargain. Not only can you find fantastic places with entrees from $5 to $ 7 to $10, when it comes to drinks and desserts you don't get blasted like you do in the US. If you look around downtown, Santa Ana, Rohrmoser and San Pedro, you can find some really great places to eat where the meal for 2 people will come to less than $25.