I think its silly to even attempt to build a system by allocating a percentage of funds to each component and then trying to stick to that number.
A great system is only as good as its weakest link! Therefore you want to select components that work well together and not by their price tags.
Put me in the camp though that believes in selecting the speakers first and they must be heard by the intended user. Then select an amp that that will properly drive them. I have seen too many people buy an amp only to find out later that it cannot adequately drive a pair of speakers that they auditioned and fell in love with.