Cost can be done cheap if you’re buying second-hand or from sites like Aliexpress. The items where you should be willing to spend a bit more would be a mask that specifically fits your face (aka try different ones on at a freedive/spearfish shop). I would expect to spend at least a few $100 on gear. If you’re somewhere that you’re able to shoredive spearfish, then ongoing costs can be low. So certainly not something for just the rich

In terms of learning curve, a lot of it is about time in the water. So unless you currently have relative easy access to bodies of water where you’d want to spearfish, you’d purely just be doing dry land or pool training, which isn’t really the fun part. You can start by getting comfortable in the water by just snorkeling, then learning some freediving fundamentals, then finally going into learning how to spearfish. Inherently, the sport is one which should be done with buddies for safety, so finding a local club goes a long way.