Just like the title. Is it worth it to buy separate instruction books and stickers to a set if for a reasonable price?

probably depends on which set you're looking at.  sticker more often then the instructions would help.  I'm thinking the cost would eat up a large part of the profits, then again I have no data to back it up 

I take it you mean in order to add to the parts of a used set to make it complete?  If so then it will completely depend on the set in question.  Just do the research to see if set without stickers/manual sells for more than set with stickers/manual minus the cost of acquiring them.

If you're talking used sets, it really is set dependent.  For buying stickers/instructions for investment, ala Cafe Corner instructions that are $100+ there's really no telling what makes for an expensive instruction book vs. cheap, outside of rarity, which you don't know on new sets. I think those days are behind us anyway with so many more paying attention to those things now.  Also, buying trademarked stickers/instructions might be impossible from Lego unless you already have the set, or know someone that does for the code number since they will probably ask for it, same as buying trademarked figures from them.