I gave up on the prefilled pens as they only dose in 1 unit increments. Great way to start but if you want to fine tune, it gets tough. I have a digital pen from Korea that dose in 0.1 unit increments and has all sorts of fancy bells and whistles (Not FDA approved but uses the cheaper Humalog cartriges). Yes, medicare does cover the cost of prefilled pens although I can buy them for less mail order in Canada than my copay which allows me a very cheap medicare drug plan (Part D $15/month).
The beauty is that if you inject 3 or more times a day and currently test more than 4 times a day, basic medicare will cover 80% of CGM costs and Part B supplement will cover other 20% and with a CGM you can really fine-tune your plan over about a 4 month period.