How do you pay a single one-time payment at a per month rate? How many months do they charge for the one-time payment?
Usually a 'single one-time payment' would be a payment for life. Typically for apps they're <$10 for the lifetime of the buyer (or app if that dies first; there are issues around this sort of pricing, but people are expecting a minimum of several years).
What you appear to be describing would be a pre- payment plan at 80¢/mo.