For providers like us, we have to lease IPv4. We came long after IPv4 was already depleted. IPv4 prices did go down. Despite that, the $15/year 128MB BuyVM plan is long-gone.
But for a new provider like us, we'd have to spend more than an established player like BuyVM or RackNerd who bought most of their servers pre-AI-boom.
For countries, if you meaning connecting to VPS, lot of countries have good IPv6 connectivity now.
For me both ISPs I use have native v6. This will differ from person to person.
It is inconcievably stupid that github, run by a massive tech company like Microsoft, has not migrated to ipv6. They're single-handedly holding back adoption.
Responsibility and controls. If the host/dc assigns a dedicated addresses the contract can be essentially "the customer assumes all liability behind traffic". With NAT/LB you need at the very least quite robust, evidence-grade monitoring mechanisms tagging all traffic and keeping historical data. In practice, some for of active abuse prevention is required, otherwise huge chunk of your address space is going to effectively linger in blacklist limbo.
That is, if being unreachable below "presentation layer" is acceptable in the first place, but I guess the question kind of presupposes this.
But for a new provider like us, we'd have to spend more than an established player like BuyVM or RackNerd who bought most of their servers pre-AI-boom.