Hi! I hope this isn't off topic, but I had to ask; would it be more cost-effective for me to replace the parts of an old laptop rather than buy a new one?

I just got an old lappie from my family, and I figured I want to make it faster for assignments and such. I think its display is 14" too, so wouldn't it be cheaper to just upgrade the parts within than buying a brand new 14"? Please enlighten me.