About $3/mo. But for a lifetime deal you're also buying the risk. If they go bankrupt, stop honoring the lifetime deal, or any variation thereof tomorrow, you're out $750 - lifetime deals, where they exist are often heavily discounted compared to normal rates due to this. 20 years is though quite a long time. Plex is only 16 years old.

In a perfect world a company would limit the amount of lifetime deals available and only have them in the beginning to get some quick cash allowing them to scale. I don't think Plex is running a very good business, which also devalues the lifetime deal.