The more you spend, the more you get.
$100 unit-single pre out, CD player (so you can play a CD, and connect a single amp)
$500 unit, 3 pre outs, MP3, Bluetooth,USB, parametric eualiser, ect ( so you can play a CD, MP3, Ipod, ect, you can connect 3 amplifiers, and have total control over them)
While they might not be the exact things you get for the price, it's just a really rough idea of the some of the features you would expect to get.
So that's why a few members gave you the advice of buying a unit that suit's your plans for future upgrades.E.g, there is no point in buying a head unit with only one pre out, if you want eventually add 2 or 3 amplifiers.