This is a hidden price hike. It would be more reasonable if there was a corresponding decrease in server costs.
It's not hidden, they put it right up on their blog https://aws.amazon.com/blogs/aws/new-aws-public-ipv4-address... the opening line of which is "We are introducing a new charge for public IPv4 addresses" and when it starts and what the cost is. I assume like every other AWS charge it's broken out in great detail on their billing statements and even have APIs to query costs. Usually they send an email with these changes too so if they haven't I assume they will. It's a regular old price hike but it's not a hidden one.
Secondly since "the cost to acquire a single public IPv4 address has risen more than 300% over the past 5 years", there's no accompanying decrease in server costs that would be "reasonable" to account for this. Charging for the IP itself makes total sense since that's the cost they're accounting for. If it were packed into the instance costs, then instances without a public IP would be paying for it too. This incentivises you to do exactly what they want you to do: use fewer public IPs where you don't need them. This is way more reasonable than an across-the-board instance cost bump which would be a hidden price hike. This is a bridge toll that covers the cost of the bridge by its users instead of raising taxes on everyone.
I guess you're wanting to pay the same and just distribute the cost between the IP and the instance differently? And hey me too, I love not being charged more. But they want to account for their costs without eating into their margin and this is how they're going about it. You don't have to like it; I sure don't. You can wish AWS would just keep eating the cost for you; me too! But I don't think "hidden" or "unreasonable" is accurate.