As far as I know recertification costs anywhere from 5-40 dollars depending on where it’s done and how detailed the job. It’s then usually good for another five years of use. It has to be done by an authorized propane tech or service.
From what I’ve gathered through feedback in my YouTube video’s comment section things can vary wildly from state to state and province to province so best to check with local propane professional in your area.
Personally, I decided to spring for new ones instead so I would have all new hardware and valves. We use our propane system a lot, and I prefer to err on the side of safety. The new cylinders cost me about $70 each, so not a whole lot when prorated over ten years.