I think it would probably help to take the PDF up front, do a combination of checking the DPI and page count to get an estimated word count (as OCRing to get an exact word count might be costly on your end), and then return back a “price preview” at which point the customer just pays the price to get their podcast.

Like others have mentioned, I’d be scared to accidentally upload a 100 page PDF only for it to cost me $100 without me really knowing up front.