I enjoy tennis. What I've found over the years though is that, while it can be inexpensive, it's also easy to fall into a trap increasing the expenses as you get more involved. For example, a person might start out buying an inexpensive racquet, a pair of decent shoes, several cans of balls, and just play against people at the local public courts. Depending on how often you play, that's probably not ever going to cost you more than $20-$30/mo.
But then someone you play with is going to ask you to play on a USTA team. Now you join USTA (annual membership ~$60), and you pay a set fee for most matches - particularly when you play a team at their tennis club (maybe $8 per match). And because you're playing on a competitive team and feel an obligation to try to do your best and be prepared, maybe you need to buy a second racquet in case your strings break during the match. And maybe you need better racquets than the one you originally purchased.
As you play more people from different clubs, you become friends with some of them. They invite you to play in an intra-club league where you play weekly matches at the different clubs (and sometimes at your public facility). There is a fee involved, of course, to pay for the court time at the various clubs. But it's fun, the people are great, and you don't want to turn down the opportunity, so you sign up.
Then one of these other players says to you, "You know what would be fun, we should play a tournament or two together." That does sound fun, so you do it. The tournament registration costs ~$25, and you enjoy drinks/lunch with your partner a couple of times during the weekend.
Long story short (too late, I know), I think a lot of inexpensive hobbies can easily become expensive the more you immerse yourself into it. At my tennis peak, DW and I were probably spending ~$300-400/mo on the hobby. I would venture a guess that the less social the hobby, the more likely it will be easy to control expenses (it's very difficult to continually turn down genuinely friendly offers from fellow enthusiasts).