I think you generally can't go wrong with buying what you like, especially if the price is good.
Some people put a cap on what they will spend for a certain item--no more than $5 dollars for pin, for example. I do that to a degree. I think especially if you're going to cut things apart or alter them, you probably don't want to spend a lot of money. I personally would try to spend no more than I would normally spend on one component, preferably less.
Of course, that is just theory.
What usually happens is that I fall in love with something and all my reasonable plans fall apart. Case in point: I just had an assignment to make a necklace with a set budget and I spent of third of my budget on a handmade pendant I fell in love with! I made it work--I used a very simple clasp and some inexpensive gemstones, but probably was not the most sensible decision.