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Proxima Nova and Helvetica don't look alike much:

The canonical Helvetica clones are Swiss721 (good) and Arial (terrible).

It's more a cheaper Gotham.

although if you look at the history, Proxima Sans (precursor to Nova) was released several years before Gotham: http://www.ms-studio.com/FontSales/proximanova.html

I don't know how much cheaper it is than real Helvetica. It's still pretty expensive.