We don't retain any art that someone pays for, in fact they get a file kit on disc when the art is finalized. Seps are our company property and never given out. Screens, well yeah nobody wants our actual screens when you tell them the price- an M3 with any of the higher mesh counts is what, about 150 bones.
We do offer film output, coat & burn and reclaim at 5, 15 and 10 bucks respectively. (nasty screens cost an additional six pack service charge;)) but this is mostly for hobbyists. We'll order screens and any other supplies anyone wants too if they don't feel like ordering themselves.
The only crappy situation I can recall along this topic is having one of my competitors (much bigger, different, sort of operation than us) freak that a group used us for an order instead of them and proceed to low ball the living daylights out of me for the reprint order. They took the low deal (can't blame 'em the price barely covered the shirt cost) and the other shop had the nerve to call and ask me how to print my art. I told them that they clearly must be confident in their skills given the extremely "efficient" rate they bid the job for and could surely figure it out on their own. They printed the basic logo instead. Shame cause that art was pretty sweet.
Point is, you pay for art, we provide proper files for you to do as you wish with but the files don't come with free consultation for seedy competitors on how to generate and print a proper under base or use high opacity plastisol inks, etc. If your client wants screens, make up an invoice and send them along their way.