If you buy it from Canada, it's actually cheaper than ProZinc. Here's the
information on buying insulin from Canada (most of us are buying our Lantus/Levemir from here)
Chewy has the cheapest price on ProZinc at about .25 cents per unit.
If you buy it from Mark's in Canada, even including the standard $25 shipping, it's about .13 cents per unit. (They also offer to add a cooler and extra ice packs for an extra $15 ..due to the summer heat, it's probably a good idea)
A 5 pack of pens is 1500 units which is enough to last most cats at least 1 year and some up to 2 years