Okay a few things.
1. Are you keeping the insulin in the fridge?
I know the site and package says it's good for 28 days. But that's mostly direction for humans, who will use a whole vial up before it goes bad when kept at room temperature. But since our cats use much smaller doses, we keep insulin in the fridge so it keeps longer.
A 10ml vial should last you 3+ months if cared for properly and kept in the fridge. Unfortunately you will not be able to use the whole vial before it goes bad and you will still be throwing some insulin away. You'll want to put it near the center of the fridge, not in the door. Make sure it is in a box or something so it can't roll around or fall out of the fridge. Never shake, drop, or roll the insulin and do NOT ever inject insulin back into the vial as this will contaminate it - shoot excess insulin into the air or a paper towel.
Here is the sticky on proper Lantus storage. You will want to warm up the insulin after you draw it into a syringe or it will sting your cat - just hold the syringe barrel with insulin in it between your fingers for 20-30 seconds to warm it up a bit.
2. OptiClik/SoloStar Pen cartridges are more cost effective than a 10ml vial.
The pen cartridges are basically 5 mini vials that hold 3ml of insulin each (15ml or 1500 units in total). Because they are divided into smaller containers you can use an entire pen cartridge up before it goes bad - I get 2-3 months on average out of mine.
So 5 pen cartridges are roughly a year's worth of insulin if your cat is on a reasonable dose (1-3u). They cost slightly more upfront ($225+ the USA or $105+ in Canada) but you will not have to throw away insulin and you are not completely out of luck if one cartridge accidentally gets contaminated or broken. The pen cartridges are sold with a pen device that draws insulin but you do NOT want this - you want to draw from a pen cartridge with a syringe, just like a vial, except you do NOT want to inject air into cartridges as you do with a vial, because they have a stopper that moves as you draw your dose out. Same storage rules apply.
Hope that helps to save you some money!!