Is "bike fit" through site reliable? I want to buy a bike, in this case a moutain bike, and the salesmen always ask my height, I am 1.90m, searching the Internet I heard about "bike fit", so I found some sites that through the measurements of your body suggest which dimensions Should use:

https://www.competitivecyclist.com/Store/catalog/fitCalculatorBike.jsp
http://www.argonautcyclecoaching.com/index.php?p=bike-fit-calculator

In my region to make a "bike fit" they charge about $ 600, I found it very expensive, being that I will use the bike to get to and from work (14km).

I wonder if it's really good for me to do, or trust these sites?

P.S.: Translated via google translate, corrections are welcome.
A $600 dollar bike fit would be a professional fit where the set the bike up, adjusting everything to the last millimeter, replacing components as required and making the bike as fast and comfortable as humanly possible. Such a fit will save you a few seconds over a hundred kilometers of riding. For a 14km commute it is not needed.  (Everyone would benefit from a good fit, some more so due to discomfort and bio-mechanical problems, but the gains of spending the time and money quickly diminish) 

When buying a bike, the most important thing to get right is the frame size, everything else is adjustable (with or without replacing parts). From a simple sales perspective, frame size is determined by height and will probably be correctly chosen 90% of the time using just height.  

Best thing to do is buy a bike from a reputable LBS. At no extra cost they should help you select the right bike (size, style etc), and set it up to fit you. They won't go as far as the $600 fit, but should have you riding out of the shop on  a bike you can comfortably and reasonably efficiently ride for the distances you are needing. (Things like seat height, handle bar height etc).