$50 isn't a a lot of money to many people so your product could still considered an impulse buy.
If you look at a product that runs $25/mo, a user would look at $625 one-time fee. I'd imagine the one-time purchases would be much lower.
The one-time fee strategy is a way of pulling people in cheaply and then upselling them. You will not a $25 a month product $650 one time, you make it $99 one time payment, and you optimise your funnel to upsell them within the first 4 months.
A $25 a month product will have a LOT of cancellations compared to a $5 a month product.