Digital BW, The Print

I'm trying to describe to someone (in an effort to justify the
purchase of something) the quality difference between a print made on
say an R200 printer (black only or not) and a print I could buy online
at a standard online printing service - like ofoto or mpix or
something.  Since I've never seen a direct comparison (and I don't
have one to use for show and tell) my efforts so far have been less
than successful.  

Can anyone quantify the differences in words?  (I realize this may be
hard to do)

Thanks, Brian