alt Hacker NewsWow! That's expensive. In the US it's certainly quite a bit higher than it was when I was a kid, but right now stamps cost $0.78 per. I think that's going up a few cents later this year. Just looked it up; in the 90s when I was a teenager, it was between $0.25 and $0.32, so it's grown faster than inflation, but still feels manageable.
Those are USPS domestic stamp prices, an international stamp is currently $1.70.
While a domestic pen pal probably isn't uncommon, the programs I recall tended to match you with someone international.