The initial cost for the pens is about twice that of a single vial, however you get 5 pens in the prescription. With a vial, you will not use all of the insulin in it. It will only be effective for about a couple months and with most cats, that means that you will use about 1/3 of the vial before you have to throw it out. With the pens, because it is packaged in smaller amounts, you should be able to use almost every drop. For me, I am able to use a single vial for about 2 1/2 months before it is empty. So for about twice the cost of a single vial you will get about a year's worth of insulin with the pens.
You will use regular syringes with the pens that you would use with the vial. Hope this helps.