Unfortunately, it's quite difficult to get a decent DSLR camera for under $500, let alone the lens for one. Basically, like the LCD panel in a HDTV set, the large sensor makes up most of the cost of a DSLR or any other camera with interchangeable lenses. This in turn requires large glass lenses to properly focus the subject on to the sensor and it's these lenses that make up most of the cost of the zoom lens, especially super-zoom lenses.
I think your best option for a super-zoom would be a bridge camera, such as the Fujifilm such as the HS10 or the Fujifilm S200EXR. These have smaller sensors, which means they can achieve long zoom ranges with much more compact and cheaper to produce lenses. The main catch is being fixed to what ever lens capabilities the camera has.
As far as I can tell, the Fujifilm S200EXR is about as close as you can get to DSLR quality and handling, but at half the cost of buying the cheapest DSLR and equivalent superzoom lens. This camera has a 1/1.6" sensor, which is larger than that on most high end compacts, has a zoom range equivalent of 30.5mm to 436mm and has optical stabilisation. It looks very like a DSLR camera and would be easily mistaken without close inspection. Like a DSLR, it has a full manual operation mode and the lens zoom ring works just like a DSLR zoom lens. This camera is priced at $439. The nearest superzoom lens equivalent for a DSLR is the Tamron 18-270, which costs at least $619 and that's just for the lens!
If you're really looking for maximum reach, the Fujifilm HS10 is another option. It is similar priced to the S200EXR, but has a longer zoom range of 24mm to 720mm. To get a lens that reaches this far on a DSLR, you're talking about a grand or two worth of lenses to cover this range. On the other hand, this camera a much smaller sensor and thus its image quality would be more like that of a compact. But if you really need the reach, I don't think there is a better option for under $500.