Programming

Coldfusion (Adobe) comes to mind. There is an open source CFML clone or two, but the real deal is a bit pricey. It's mostly used by government, higher ed., and healthcare. It's not terrible, but it was cooler when it was younger IMO.

If you invent a language and it takes off, you’re literally the expert on it and will reap the financial windfall of that.

That sentence seems correct. As for your question, you'd rather have to ask yourself: When would you pay for a programming language?

Back in the old days before internet and commodity hardware, before easy distribution, before non-trivial software, before basic compiler knowledge was common people were easily impressed, so naturally you could sell your compiler for primitive grug BASIC.

Today, that situation is different. A language by itself is meaningless, a risk, even, in some ways. A while back some developer of REM Objects was bitching about in the old aggregator on how nobody wanted to spend money on their special proprietary BASIC. Like it was just another product. They don't know ~~how to use a computer~~ how programming languages work. This extends to even finding users willing to use your stuff for free. I don't want to knock on REM Objects in particular, you know you could add way more examples like Seed7, every lisper ever etc., but let's get into why people pay for programming languages.

• extreme vendor lock-in of platform, hardware, whatever (but you almost always have your language bundled with something else in the first place, because...)
• the point is a feature beyond the language (think matlab, labview, SAP, Excel or Coldfusion 8bitguy mentioned)
• dubious support contracts (think Oracle JDK)
• enthusiasts you can milk for life support and dumbfucks that fall for your next gen graph AI DSL whatever bla
• extremely great developer experience (but we live in an age where megacorps create languages and infrastructure for improved experience for free, so, good luck with that these days)