Rust was Garry Newman's second game, following along from the hugely successful Garry's Mod, which sold millions of copies over many years. Despite not offering much in the way of features but an open world and some crafting and even though it was rebooted a year in and turned over to a new engine with even less going on, it's managed to sell over three million copies.
Considering most of those went at $20 a pop, Newman and Facepunch studios won't be wanting for money any time soon. It was already well funded by sales of Gary's Mod, so the fact that Rust continues to do well as an exceedingly early access title shows that it should have no difficulties in funding continued development for many years to come.
You could argue that most of those sales may have come when the game was brand new and there was that huge influx of players, but Rust has continued to sell well over the past year, totalling more than a million copies shifted in 2015 alone.
“We entered 2015 with the reboot in its infancy,” Newman said in his year review. “We were seeing an average of 6,000 players online, and peaks of 11,000. This was a low point for us, but we had accepted it as an inevitability. We are never going to reach Rust’s initial popularity.”
However Rust is more popular now than it was then, with an average of around 15,000 players online at any one time and peaks of close to 29,000.
That's a pretty decent player base considering AAA titles quite commonly drop out of those sorts of numbers after a few months as people lost interest.