Considering that 1gh/s equates to bout .75 coins a day and will only get worse as difficulty ramps up you're probably better off just using that money towards investing into bitcoins themselves. Considering that you're willing to spend at most 850 for a 1gh/s rig you could spend that towards about 55 bitcoins at its current rate. That would take a 1gh/s rig over two months at its current difficulty. That's assuming you don't think the difficulty won't go up...which it will.