dual stereo usb soundcard options under a hundred bucks are
Numark DJ I/O Interface
MixVibes U-MIX 44
Native Instruments Audio 2 dj

there's also the option of getting a standard usb soundcard for 20 bucks and aggregating the soundcards (so they appear as one on your mixing software and you're free to route the signals whichever way you want monitor/master output)