What is your "inexpensive" hobby that turned out to be expensive/ you gradually invested lots of money into?

Traditional painting and illustration! While I now know that I never needed to spend more than $250 for professional-grade tools, I've spent about $18,000. As for sales in 3.5 years, they don't account for more than $800. For that I mostly blame Instagram where it's not possible to grow anymore organically and get an audience & potential customers. So I moved to the federated open source PixelFed now, if anyone's interested in my book-style illustration: https://pixelfed.social/EugeniaLoli

Also, as a word of advice for anyone who wants to also do illustration and don't want to do the same mistakes that I did. All you need is:

• The Lukas 24 watercolor palette of student grade ($18). It's good enough and these days most paintings are scanned, so even if not all colors are lightfast, it's not a big deal. Few people only buy originals, most go for prints. If you're going to go selling originals, consider the Daniel Smith primaries set of 6 colors for $40.
• A set of brushes of different sizes, including a flat brush and round brushes including a long thin one to do details, $15
• Pencil, eraser, sharpener, $15
• A set of gouache. Best bang for the buck for professional quality is DaVinci brand ($10 per large tube), or if you want to go cheap, the Himi Miya set for $25. If you go for the cheaper stuff, it's still advised to get a better quality white tube, so it's truly opaque (the cheap stuff aren't opaque enough). So go for Holbein or DaVinci white for $10-$15.
• Soft core colored pencils, set of 48+. $15 (you will mostly need the muted colors to enhance the painting with harder edges)
• Grey, sepia, black ink pens, and manga ink brush pens (for some types of paintings only), $40
• 100% cotton paper for watercolor $25, or any watercolor paper for gouache $10 (gouache works on any, watercolor is more nuanced).
• Brush watercolor markers, e.g. Tombows or Ecoline -- in case you want to do such type of illustration too, $30 for a few muted colors.
• Masking fluid for watercolors, $10
• White gel pen and white Posca pen (0.7mm) for white highlights, $15
• Faber Castell white pencil soft pastel, $4
• Caran d'ache Luminance white colored pencil, $4 (the cheaper colored pencils above again don't include a strong white)
• Caran d'ache Neocolor II white crayon, $4
• A ruler, to help you sketch.

I included various mediums above in white color because highlights are king in illustration, and each provides a different look and feel, depending on the painting. Happy painting!