Knitting. Super cheap to start, you can pick up a set of needles and some acrylic yarn for under $20. But when you start getting into nice yarns and bigger pieces, you are spending hundreds of dollars on yarn alone for a blanket or a sweater. And you want nice needles in all sizes as well as all types (double pointed, regular and circular)… more hundreds of dollars.
Moral of the story is if a friend knits you something with nice yarn, please appreciate it. Lots of effort and thought went into it.