I was also given an AccuChek Mobile but no prescription for replacement cassettes, and yes, they are outrageously expensive, so I also bought the Codefree from Home Health. There is also a discount code for a box of 10 tubs of 50. 10 packs 975833.
You can also get the AccuChek cassettes very much cheaper on Ebay - just watch for reasonable expiry dates. I keep a continuous supply of cassettes as emergencies should my Codefree die, and to do the odd comparison checks.