Im not sure if Im following you...but your trying to buy your old screens from your old printer to give to your new printer to use ?
Did you purchase the screens or the set-up fees associated with printing from your old printer ?
Most printers charge a fee to get the screens prepped and ready for printing.
If you have no pre-existing agreement on the subject, I tend to think they can do whatever they want.
If there's something special about the screens, or the screens mean a lot to you and you can recoup the money, I'd buy the screens. If not, I'd have your new printer burn the art again, because $50 per screen is fucking ridiculous.
Your new printer would probably rather work with his own shit anyways.
I am part owner of a screen shop and understand where you are coming from.
You are not actualy "buying" the screen. It is more like you are renting it and paying for the time that it takes to set up the screen. In my shop we clean the designs off the screens if you will not be using with in a week or so. Depening on what type of screen you buy ours usualy run around 25$ but depending on the mesh count could be more like 40$. It is also a slap in the face to ask for the screens, if someone does that to us we feel like we are wasting our time dealing with people that do that. Screenprintes dont make there money off one or two jobs they make it by repeat business. So when you say you want the screens your taking business away from them and the time it took to set the screens. Cleaning a screen and preping it for a new job take about 1 hr. When you sell a shirt you usualy want to double your money, so screen printers want to make money on the screen so they double it, thus you would be buying retail. If you want to keep your screens you should just go online and buy them but then the screen printer would still have time setting it up. I would just pay for new set ups or use your old printer.