Inherently expensive: Fountain pens, nice paper, ink.
Potentially expensive: camping/backpacking, kite-flying, babywearing (don't ask unless you really want to know), parenting, sewing, judo, homebrewing
I'm poor at least until I finish school and work for a couple years, so I'm pretty cheap on even the inherently expensive stuff. I'm limited to 6-8 boxes of shells at the most per week, and right now I'm only doing about 2 boxes/week. That's *nothing* in trapshooting. Starting to reload 9mm, on a friend's equipment. Got a line on a used shotshell reloading, which won't save me much, if any money, but will buy me consistency at a cheaper price.
With pens, I'm limited to the low end stuff--maybe one sub-$100 pen per year. I do go through about ten or so Clairefontaine writing pads per year at $7 each. I sometimes pick up a low end vintage pen. Got a transitional series Esterbrook for $30 a couple months ago. Lately I assuage my pen desires by buying ink. A lot of fun to experiment with and under $10/bottle, many under $5.
The other multitude of hobbies I can do on very little money but sometimes it is fun to get some new, pricy gear. Mostly not until I'm done with school. Haven't gotten an expensive kite in over four years. I buy fabric on clearance, and sewing for a toddler uses less fabric than sewing for adults. A friend gave me a whole bunch of honey a couple years ago and that is satsfying my mead-brewing needs for a while. Did have to go real cheap in the last batch. Despite my top-of-the-line corker, I "bottled" it in Mason jars--didn't have the cash for bottles and corks. I like having a zillion hobbies and find that there are corners to be cut in most of them. There's always .22lr, there's always PR inks, there's always basic delta kites and dollar flannel and mason jars.