Although I'm brand new to the MPC game I've been a DJ for 9 years, I'm assuming you're talking about a pre-amp needed for your turntables to sample from? I'm not sure what mixers you were looking at, but any DJ mixer should have a pre-amp in them and they have a TON of mixers under $200 and even a couple for under $100. Any one of them should work. Check these out:
http://www.audiolines.com/product.php?p ... t=0&page=1
http://www.audiolines.com/product.php?p ... t=0&page=1
I would recommend showing these prices to your local Guitar Center for them to price match. You'll save on shipping, plus I have a strange feeling these guys (audiolines) are kinda shady. Every single time me and my boy has called them or come down to their warehouse to check out a "Floor Model" or "Open Box" deal they always suspicously "Just sold the last one," "Can't find it," "the only copy is on layaway" or someother bullish excuse that they don't have it and they try to sell us their brand new models. We think they're just fronting on most of their so-called "Open Box models" just to lure customers in. Either way those two mixers I found just by doing 1 search in Google, if those aren't what you're looking for I'm sure you can find something else.
Sh*t, sorry so long but I just remembered that any portable turntable has a decent phono pre-amp built in them:
http://www.topdjgear.com/nuptpotu.html
http://www.topdjgear.com/vehatrpotu.html
They ain't the best but they get the job done. I remember hearing that J Dilla used one of these and like a Boss Sampler or something to make Donuts on his deathbed. If you're lucky you can find the Vestax one in Black for clearance at an Urban Outfitters (I got mine there for under $50.)