I think it depends a lot on what type of music you listen to, and what comprises the rest of your gear. I'm also assuming you're talking about a 2-channel setup. Total budget would affect the answer as well. As a general answer, though, if I were setting out to buy a new system today I'd spend 40-50% on the two speakers.
Perentage system $$ for speakers
While reading various posts in the past, I have read comments where some believe that the speakers should only be about 15% of the total system cost where other believe they should be the main focus. I am not looking for a right or wrong answer in this, I am just curious as to the thoughts of others on this topic.