I was staying at a Hostel for $10/night US inc breakfast in Alajuela. Food is a bit more expensive, like you said, if you buy import items specifically. Snickers were like $4 (in 2015), and a 5th of Jack Daniel's was nearly $80.

But (at the time), eating like a local was very cheap. Rice and beans aren't expensive. I was buying whole roasting chickens for $3 US. Bread was pricey then, too. Nuts were also shockingly cheap, as were exotic fruits. Perhaps not at the mercado in town, but roadside? Local? The Costa Ricans aren't idiots and you'll pay a tourist tax there if you stay in tourist spots.

I stayed in Costa Rics for 3 months, my entire length allowable, and I spent $2000 US including airfare, passport fees, and I lived luxurious like smoking weed (VERY EXPENSIVE). I stayed between hostels, and worked a bit on a farm. I guess depends how you do things.. but CR can be done cheap. There's a reason it's the new Vietnam.