I use a pc.
You stated you are looking for USB input but if you have a digital output from your computer then tc electronic bmc-2 [DAC and monitor controller] could work for you within budget. It is surprising they don't include either a USB or firewire input on this device though probably due for a product refresh.
RME's babyface also has a similar form factor but is over your budget.
While not really a monitor controller Propellerhead Balance is another similar cross platform option that is under budget. For a different form factor maybe an Avid MBOX could work, it's lower quality and offers less monitor control functions but it's under budget and offers USB input.
Looking through the new Full Compass catalog today saw a new product: Steinberg UR28M USB Audio Interface.
Can't really endorse any of these merely presenting some options that might work for your setup, good luck with your search.