Basic Lego is actually decently affordable. It's the collector's sets that adults would buy whose prices are jacked sky high, based on demand it seems.

I've bought a decent amount of Duplo and Lego kits for my son (currently 3 years old) and it's great value.

The fact that "10$ a pound for used, 10¢ a piece for new" has remained true as a good rule of thumb for 20+ years tells you something.