I have to tell you, in the end, you will need to just get over the feeling of being inefficient or costing money in your diabetes care. It is expensive, and stuff will get wasted. But if you do everything in your power to be efficient and not waste, then don't waste your worry on it.
That being said, I have even more alarming news for you, Humulin 70/30 (or Humalog 75/25) mix pens are recommended to be used within 10 days of opening.
You can keep them in the fridge, and that will slow at least some of the degradation, but insulin mix in pens lasts the shortest of all the supplied insulin forms. Some insulin formulations like Lantus and Levemir actually want you to inject at room temperature, it helps them work properly, but I don't think it matters with Humulin/Humalog mixes. If you do need insulin, you will likely start to need more as 1 unit in a basal form is almost nothing. And in the meantime, one option might to simply start using NPH overnight (it lasts 28 days in a pen).