I would buy them something but I would never simply give them money. You give them money to use wisely. Say you give them $100. What if they spend it on something insulting? It is just better if you purchase something for them yourself. If you are their friend, then you should know what they want.
I would only spend a couple hundred on them. My best friend loves weapons, so I'd get her a few nice knives, maybe a hawk axe, and possibly a machete. For my boyfriend, I would get him a new motorcycle.