To be honest he's not wrong. I bought my trad rack (widgets that go in cracks and carabiners for them) for around $2,000. Rope, shoes, helmet, harness, backpacks, etc on top of that is probably another $2,000 at most. Camping gear is another ~$1,000.

Most of this stuff (aside from the rope) will last for many years. Occasionally I'll lose a cam out climbing which is $50-80 to replace, but only happens a few times a season.

Compare this to skiing or mountain biking and you can see how climbing is a vastly cheaper sport.

Spending 5k on a sport is expensive for most people.

what materials do are you thinking about when saying climbers leave things? If you do it correctly you leave with all the gear you came with

Not always.

Sometimes you have to sling a tree or rock in order to rappel. Sometimes gear gets stuck and you have to move on. Sometimes you drop gear by accident.