i buy cartridges 100 at a time for $50 shipped to USA from china, so $0.50/pc. once filled, a cartridge will support anywhere from 10-30+ injections depending on the peptide and concentration/dose. Pen needles are cheaper than insulin syringes. So at the end of the day it's not like it's a high cost endeavor, perhaps adding a few cents per injection.
there is definitely a learning curve to filling them, but at this point it only takes me a few minutes and like i said only happens once every 10-30 injections. for me the hassle trade-off is well worth it because i am taking a couple of injections per day, which are significantly streamlined by the pen. If it was one per week I would definitely not bother with pens.