You can even find something for 0 yen.
The big question is whether you will be able to find tenants for such remote properties.
Yes I think we're both in agreement here (and also with
@Petaris above). If the OP is looking for a bargain property, he would do well to browse some of the "akiya banks" of towns that are offering distressed properties for free or for very low prices. And certainly any real estate, not just distressed properties, becomes super cheap the farther away from Tokyo you get.
The problem is that the OP is looking to generate rental income. So the first hurdle is getting a property suitable to live in himself, plus one that is suitable to accommodate paying lodgers. Many of the free houses or low price properties come with structural issues and conditions. This could eat up a large chunk of the 12M capital he/she already has. Let's say it would take 5M JPY to bring these places to a livable condition.
The second thing is that these kinds of properties are often
way off the beaten path, and far from public transportation, which means they will be tough to rent out. Now I think there might be an opportunity to offer a super remote place as an "authentic inaka Japanese experience" type of accommodation, but this again is going to require significant upgrades and probably a good bit of marketing in order to attract renters. Bear in mind the OP also has to live here, so the more remote it becomes, the more potentially difficult and expensive it becomes to live. I'm thinking about places like the Iya Valley property that Alex Kerr rents out. Its a lovely-looking place, but man is it ever remote. And of course its been vacant for a while due to covid, so the OP also should factor in a certain amount of time when the property will be vacant and there will be no income coming in.
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And taking another step further in this analysis: the farther away the property is from public transport, the less likely it will be able to be let out as a residence, so it becomes exclusively a vacation type of rental. Nobody wants to live in a place that is isolated from services. (People live in isolated places when they work the land, but not typically as a lifestyle choice unless its a very specific lifestyle). So here too I think
@Lomaster and I are in agreement.
And just another thought on this for the OP, its important to figure out how much income you need to live in the way you want to live. How much will you need for insurance, property tax, repairs, etc., in addition to how much you'll need to support yourself. I'm just spitballing here, but I reckon you'll need at least 500,000 a month - so you have to kind of reverse-engineer the plan by figuring out what kind of properties can generate 500,000 yen/month in rental income.