Make sure they include a magazine. The 5.7 mags are a little pricey. The knockoffs do not work as well as the FN factory mags. Sometimes you can find a deal with an extra mag or two for the same price. A lot of people buy them and decide they are not what they are looking for, so you may want to look for a used one. There have been several used ones for sale, one had 4 extra mags with it, and bunch of ammo. $600 seems to be the going price.
Independence Firearms had one yesterday. It was $900-$950 upper and lower. Had a 50 round FN mag on it. Didn't notice the brand of the lower...but it was a complete kit.