Unless you want to spend all afternoon with bubbles and sidewalk chalk, a lot of adult hobbies are pretty expensive (Might I direct you to the hundreds of dollars I've spent on scrapbooking stuff?

), but it doesn't always have to be that way!
For the cost of shoes, you can take up running. Well, maybe a good bra would be a worthwhile investment... anyway, for $150 or less, it can be six months or more of passing the hours. At the runner's store, you'll find tons of meetup groups from beginners to triathletes. New friends with the same interests will make it a lot more fun!
Have a love of music? Blast it out and just dance. If you're serious about learning new moves, youtube is your friend. But if you're like me, you just want to cut loose without the idea that anyone is watching you.
Do you love poetry? Short stories? How about writing your own! Write a blog- if you've got a great subject matter, people will flock and you never know- you might get some sponsors!
Teach your old dog some tricks.
Go for a hike.
Take a bubble bath.
Practice new hairstyles.
Sing really loudly.
Learn a new skill- knitting, an instrument, cooking something new.
Head to a new town and window shop.
When you open yourself up, there's limitless things to do that won't break the bank!