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That's not typically where the software industry sets the one-time purchase though. 18-30 months is more typical. In your example that would be $5/mo for software costing between perhaps $100-150.

If it's set correctly, and the software is actually valuable enough to be worth something, there's a more balanced tradeoff between the risk of overpaying and underpaying and the benefit of the subscription acting like an inexpensive trial that will likely convert to a one-time purchase if the buyer is budget-minded.

(That said, it's certainly true that you can approach the same pricing strategies fairly or greedily; I've overheard both.)