wow! that is expensive! My first ISP charged $5 a month. It was DSL. I switched over to a different ISP so I could have fiber and they charged $20 a month for the static. (Free modem and router too 
) There service was slightly less than the DSL provider, so after static IP it cost me like $10 more than I was paying. So… $80 a month for 100mb fiber.
I know nothing about duckdns, but I was impressed with how easy it was to setup dynamic dns using google domains. Like I said, I’d kinda be concerned about security… but of course it’s up to you and if it’s an acceptable (if any) risk. Maybe do some internet searching and see what is said about cyber security and dynamic dns… Then use that to make your decision.