Mastodon org + Mastodon.social also have costs of 0.6 EUR/year, though they have two orders of magnitude less users [1]. This is really what most social media costs. These rates are even payable by many in poorer countries.
With how much Mastodon.social tends to fall over when Twitter does something stupid (again), their rates are probably a bit too low for a more robust service like Signal.
Signal also intentionally doesn't store too much data, long term data costs will slowly grow over the years. I imagine for a bigger platform, costs can grow to multiples of the rates for Signal and smaller Mastodon servers.
€10 per year should be more than enough for most users, though, and it should be quite affordable for most countries.
Yeah, the issue is more that there is substantial friction in paying any amount of money, especially in poorer countries with no access to e.g. banking or payment cards. I'm sure no one here, and few people even in comparatively poorer countries, would object if Signal/their messenger of choice cost 0.60$ per year to use. The problem is that making the service have a ~1$/yr price tag (as WhatsApp once had) is itself a barrier to a huge portion of the target audience.
In Pakistan at least, sometimes you can donate to charity etc by texting a special number [1]. That subtracts some fixed amount from your prepaid mobile balance (which the vast majority of people use) or adds to your postpaid bill. I imagine its possible for some business to charge customers this way as well.
Then again, instant C2C and C2B digital payments using mobile phones is growing extremely fast in most of the global south.
[1] https://news.ycombinator.com/item?id=38117385