Less than $100 greatly limits the options unfortunately. The numark one I linked to runs under $100 (barely), but seems cheap to me, but I've never seen one in person nor do I have any experience with it.
I'd be tempted to say try an iMic and create an "aggregate device". I know that's the case in leopard, dunno if it's the same in tiger.
[update: looking at the numark DJ/iO on their website--it still looks plasticy, but it does seem to go for the price/basic needs point that other options are missing. if an iMic doesn't meet the needs, this may be something to try.]
I'm not a professional DJ, but to me the "digital revolution" doesn't seem much different than DJing with CDs--if anything, it's simply more convenient and with a greater wealth of songs at your fingertips. When I DJ publicly, I do vinyl, but I'd like to keep a laptop on hand for specific requests.
i.e. on my radio show tonight my cohost and I were talking between sets and he mentioned something that got me to almost instantly bring up a related song on my computer and completely change where we went. A lot easier with a hard drive than a crate of records or box/binder of CDs.