How reliable are QS Top Universities rankings? I was told that universities in USA are generally better that the ones in Italy. I will take the undergraduate degree this July, and I need to decide whether to continue my studies in Italy or to move to the USA.

This site reports the world ranking for computer science &amp; information systems, and Politecnico di Milano is ranked 48th.  

I'll not be able to afford an university with high tuitions like MIT (let's say 15,000 $ per year maximum), also since I may have not the prerequisites to join high selective universities, if I move to the USA I'd choose a university that is ranked below or almost as Politecnico di Milano. So the indecision: it is still worth to move to the USA? Do you think that the rankings are reliable?
I was told that universities in USA are generally better that the ones in Italy. 


This statement, in its given generality, is false. The US, like most places, has very good schools, ok schools, and a reasonable number of pretty bad schools. If you explicitly are looking for very cheap schools, I would not assume that the remaining selection is necessarily any good, just because it is a "school in the states".

POLIMI is a pretty good school in CS. To be honest, if I had the choice of going either to POLIMI or to a low-ranked US school, I would almost certainly choose POLIMI.