I'm interested in finding out how you can choose a good master and where is better,UK or USa? Also,not being interested in studying at the bset University,necessarily,still Im searching for a master that provides a balance between ost/time/quality.

Do what interests you, as long as you can afford it. Study is about passion as much as cost. As long as you find a reasonable quality university to study the subject you like, you'll do fine

I'd say UK is better. But I'm no expert..

I prefer UK because it has lots of well respected universites and because graduate study here is a fraction of the price of the US.
although there is no such thing as an absolute measure of world rankings for univeristies, look at the times and shanghai rankings which generally fills the top 10 spots with either UK or US universities
personally i dont want to spend 6-7 years doing a PhD in the states after which id be left without a penny to my name when i could do one in half the time in the UK without putting my self into student debt.