There is currently a gap between about $70 [ASUS Xonar DX] and $120 [ASUS Xonar D2(X) and Auzentech Meridian 2G], though you can sometimes find an ASUS Xonar D2(X) for about $100 or so. For most people, the DX is more then sufficient to meet your soundcard needs.
Note though, if using [or plan to use] high impedance headphones [<150 Ohms], the ASUS Xonar DG may be a better bet.