Yes 50mm is a good versatile lens it was the first lens I got when I was using pentax but haven't got to it yet, its a good length on full frame as well your right as for cost, depends on which one you go for there are a few from $300 up to $1500 I think for one that is F1.4
I am going to have a limited budget to but lenses up to a retire so are debating between a few options