We had a really good post a couple of years back on the topic of saving money. I cannot seem to find it.
1) Buy lots of fruit at the market for snacks and at least one meal. Bring instant oatmeal with you for breakfast (healthy too!).
2) Yes, call 6, and get numbers of chicas willing to do TLNs. You can probably have for far less than half.
3) Buy all alcohol at duty free. Carry a flask with you to different venues you go. Get a class from bartenders and pour your own shot in the bathroom (I do this becuase I like high end hooch you are not going to get in SJ, but it works for saving money as well).
4) Take the Interbus from the airport.
5) Castillo has good rates for the week if you are staying that long.
6) Make a list of free activities you want to do/see and actaully DO them. Engaging in some inexpensive non-mongering activites saves money and makes you feel less like a slime

(i.e. a museam or two, mercado central, take a bus to one of the towns in the valley and walk around, i.e. Alejuela, Herredia, Cartago).
7) Have some activity you want to accomplish in some of your down areas (reading a book, writing letters to friends you have not had contact with in some time, ect).

Pack carefully and wisely. Really take whatever you will need so you don't buy things.
9) Ask yourself if you can do without X and still have just as good at time.