When you make inquiries about a new set of tires, it is a known commodity, and price differences will show that. You are comparing apples to apples. When you engage a number of arborists to bid on pruning it is more like comparing apples to refrigerators. The main things you buy from an arborist are attitude, experience and knowledge. Every one knows approximately what the the job is worth in dollars, but when you compare attitudes there will be a great difference. You are not buying a set of tires, but trying to help a living being. If you could magically have 20 arborists prune the same Tree, some of the results would be beautiful and healthy, some would have received more damage than they should, and some would be hacked away enough to seriously threaten the Tree's state of health.
Most of these jobs would align in a very competitive group pricewise. Price is no indication of quality. Price is not a good way to pick the person who is engaged to work with something living, something where their actions can do good or harm.
Whether it's your dog or your Tree that needs help , use your gut to help make your choice. Someone who is there to help, with the right energy, feels good. Someone who isn't, doesn't.