I don't usually buy the niece and nephews real presents. I give them candy or a fiver or something. Maybe when they reach adult-hood, I'll start handing over things for their first apartments (dishes,silverware, old furniture) and be the cool giving Aunt and what-not, but for now, no Christmas presents.
Why don't I buy the kids real Christmas presents? Because they get way more then enough from their siblings, their parents, their grandparents, and their extended family, blah blah blah. They end up with a lot of stuff every year that just accumulates into a heap of junk that ends up in the garbage or in a yard sale.
Also, there are seven kids total.
I am not buying seven individual Christmas presents for seven kids.
Especially since most of them are in their tweens now, and will *sigh* at whatever I give them anyway. Their parents have made them impossible to shop for. They all have cell-phones, and everything 'cool' already. And if they don't, their Mother has this, "If you buy for one, you must buy for all." mentality, and she can go fudge herself if she thinks I'm going to buy five I-pods (or equally priced item) for her kids.
(I know what you are thinking, "GROUP present, genius. Was that so hard?" I tried that. I wont again.)
I only buy for the adults, and I really think about what I am getting them that they will be able to keep, use, and enjoy based on their personalities. They work really hard all year to provide for their families, and deserve a few nice presents (more then they usually get anyway). Especially since nearly all their money is going into those life-draining demon spawn.
EDIT: My cousins and I are all of the same age, and don't see each other very often. Once a year if that - so no, I don't generally buy them gifts.
I ended up ranting, when I could have given quite a simple answer: Really, it's all about affordability.