Almost everyone I know who considers music a hobby ends up spending a pretty good penny on it once they really get into it. It can easily be one of the most expensive hobbies someone has (it is for me at least). Once you've learned the basics, half of the fun is collaborating with others or recording.
Costs:
- Instruments & accessories (serious guitar players often have at least one acoustic and one electric - and good quality ones are going to be at least $500, but could go way higher. Other accessories: at least a metronome and tuner, potentially also some pedals, amps, drum machine, etc)
- Renting/owning/traveling to a practice space if playing in a group
- Educational material: Lessons, Books, sheet music, music apps
- Recording / mixing equipment
Sure you can get a used acoustic guitar and watch youtube videos to learn it - but I don't know many people who do that and stick to it.
The real up-side though, is that decent musical gear doesn't lose a ton of value over time unless it gets broken or stolen. Also, if you're not professional, you can pace out your purchases.
My personal collection grew over 10 years, and is just for amateur use, jamming with friends mostly. I've got a couple guitars, bass, keyboard, 2 banjos, amps and mics. When I add up everything I've spent, it was probably 2/3 the cost of 10 years of $100/mo cable. I wasted a decent portion of that cost on a few other instruments I never played and ending up giving away as gifts to friends though.