I did look into it and the data was mixed. Some very good results. Some less so.Good for Fret. She has been through a lot , certainly.
I searched through my emails but seems I threw away the relevant one from the ASPAS federation calling for adoption. I did find the update from the shelter that collected the hens and kept eight of those remaining. They explained that the actual rescue operation was led by another association that operates in a more classical manner, rescuing thousand of hens, and they took out of those 20. Then they took care of organizing vet visits and care when necessary, implants, and adoptions. So there was indeed an intermediate, which would as you mentioned pose a biosecurity problem, hens being moved twice.
They also said that even with the implant and vet care, one of the hens they kept died about a month after, of infection.
It's not only paying initially, but also about being able to maintain the hen on the implant if she's doing well with it both financially and taking her to the vet every four to six months.
As for having an idea of the life span gained by suprelorin/ deslorin implants.... not sure there is enough data yet to know. @RoyalChick did some research and could maybe say. I have been following rescues who use them, with the impression that most of the time they are used once a hen already has reproductive issues, so they will extend life for a year, maybe two, but not "save" it. The person I asked about it said although there could be secondary effects in some cases, it was worth giving them a try, and sometimes it could help a hen through a difficult pass and then the reproductive system seems to get back on track and she can get off it.
For those associations that are operated by "anti-specists" (sorry, I lack the vocabulary to express exactly my meaning) I believe using implants is done systematically, almost as an animal right gesture, to allow the hens to lead a life without having to lay.
However in the course of doing so I found a fair bit of data on using progesterone-only oral contraceptives in chickens and that having very good results and very safe.
They are also available over the counter in many countries now.
So I decided if I ever wanted to suppress ovulation in one of my chickens I would go the oral contraceptive route rather than the implant.