"How much people are willing to pay for an unfinished game in development without any update coming any time soon"
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Yeah, everyone know the answer, nobody. Why pay for a game that has no updates, instead of paying for those that do ?
When I start supporting a developer, if I don't see regular announcements or updates within 2/3 months of the last update I stop.
Now what if you do regular update, and set a reasonable price on it, well, do it. People will pay for a good game, and updates.
BTC have a great story and many ideas to continue, if you want and can do that, add a sandbox mode.