You're probably going to have issues finding a mini fridge with enough floor space to fit a carboy. The issue is that the compressor takes up so much room at the bottom. Your best bet is to look for either a 5 cu ft +/- chest freezer or a counter height upright freezer. The chest freezer you can typically find for around $200. You can probably find a Haier, GE, Kenmore, or Whirlpool in that range.