If you're looking for a straight up varminter, check these out. Come with everything you need in an upper, sans sights, ready to be slapped onto a lower, and all under $600
http://www.midwayusa.com/viewproduct/?productnumber=173618
http://www.midwayusa.com/viewproduct/?productnumber=224299
http://www.midwayusa.com/viewproduct/?productnumber=496810
http://www.midwayusa.com/viewproduct/?productnumber=519542
http://www.midwayusa.com/viewproduct/?productnumber=799855
http://www.midwayusa.com/viewproduct/?productnumber=173618
http://www.midwayusa.com/viewproduct/?productnumber=224299
http://www.midwayusa.com/viewproduct/?productnumber=496810
http://www.midwayusa.com/viewproduct/?productnumber=519542
http://www.midwayusa.com/viewproduct/?productnumber=799855