Until a few years ago the cost to get additional IP addresses on most business ISPs was pretty minimal. The ISP my company uses does not even charge for additional IP addresses, you just need to give good reasons for why you need a larger IP space. With a previous ISP, additional IP addresses were about $10/mo but the cost of service was easily 3-4x similar rated speeds as a residential connection. Most of the additional cost is due to increased capacity planning and faster service times.