How much would it cost to hire a private tour guide per day in Costa Rica?

That depends on the tour and the area you would like to visit. I would not recommend using one guide for the whole country. Local guides know their area much better than anyone else. Tour companies usually provide transportation and access to National Parks / other points of interest. Nature walks and birding tours start around $40 for 3+ hours. More elaborate tours, of course, cost more. Here's an overview of tours you can experience in Costa Rica: http://www.vacationscostarica.com/tours/

It's not a definite list, but can help you get an idea.

A lot goes into this. Just depends on who it is, how much they know, and how much they want. You can hire people all the time for stuff like this. It's really a shot in the dark on how much. Some will charge you $50 a day, and some could try to charge you $200 a day. Depends on where you are at also. It would be cheaper to find someone from a small town and away from the tourist areas. Always cheaper in those places. Also depends on the transportation, gas is over $5 a gallon here.

Thanks, Ana! :)