Generally, you pay things like the cost of spaying or neutering and vaccines. We paid $150 when we adopted our last dog to cover those costs. $150 is a hell of a lot less than over $1000.
My local shelter has free adoptions. Everyone returned the dogs they bought during COVID, so they just need to get rid of them as fast as possible from the shelters.
Yes generally, but adopted animals can absolutely have challenging behavoural issues that not everyone is suited to deal with. It depends on your situation I think but absolutely more people should adopt.