I probably would pay to visit an unfinished theme park, but mostly because I can't imagine a better death than roaring through a loop de loop into a six-twist corkscrew then launching up a ramp to... nowhere, nothing, unfinished track, screaming through the air then landing in the crushing jaws of a still-skinless animatronic dinosaur. I'd be up for that. Let's start smaller, shall we? You can now pay to create and visit virtual theme parks of your own in an unfinished version of Planet Coaster [official site], as Frontier Developments have started selling alpha access. It's pricey, mind - more than twice as much as the game will cost when it's done.
Frontier Developments may be known for space sandbox Elite Dangerous nowadays, but they've made oodles of games about building rollercoasters and managing theme parks, from bigguns like RollerCoaster Tycoon 3 to diddyuns like Coaster Crazy. They like rollercoasters. Planet Coaster is the first on PC to sound interesting in a fair while, though. Jake Tucker had a look at the alpha, noting that it's clearly incomplete but:
"[. . .] what's here already is a good advertisement for the game, and should give Frontier the opportunity to change anything they want in response to feedback. As someone who has long wanted to build a pretty looking theme park and is happy for now to simply slap down a few rides, this is a rollercoaster I'm already tempted to jump on board."
Frontier are now selling the 'Early Bird Alpha Edition' through their own store for £49.99/$74.99/€67.99. That'll be updated until the full game launches (at which point it'll become that), which is due to be this year some time from September to the end of December. However, when it properly launches, it will only cost £19.99/$29.99/€26.99, so this alpha is intended for serious track-heads.
Here's a video with Frontier explaining the basics of starting a park in the alpha: