A budget model of a device is generally one that has nothing going for it over the premium model except price. The iPad Mini has proved, however, to NOT be inferior to the full sized iPad when price is taken out of the equation. Many people who could easily afford a full sized iPad opt to buy the Mini instead, as they like the size, and even some people who already have a full sized iPad are buying the Mini.
However if Apple brought out a budget iPhone, can you imagine anyone that already has a full sized iPhone, or could afford a full sized iPhone, buying it? I can't, unless it's just to review/try out the device.