What you're getting is a solid base, dovetail ways and a decent spindle for about two dollars a pound. Of course first thing when making it an nc is to throw out those screws, and most of the conversion kits have different grades of ballscrew as options. I can't advise on the low end, sorry.
Personally, I use Bosch-Rexroth precision rolled ballscrews. If you go to my site, you'll see my ServoNut power module. (http://bell-everman.com/actuators.html) scroll down to the picture...
Not the cheapest, but not too bad, and no where as expensive as ground, and better in my opinion. A fully internally preloaded nut, no springs, no dual nuts, just plain *****in. The rolled surface is smoother than ground. All in all, A+, and I based a successful product on it. We typically use Renishaw linear encoders (I'm not proposing that DIY'ers would) but I bring it up because we cannot stand the slightest backlash when using linear encoders.
I've tested the 16mm x 10mm Lead to 300 lb accel/decels, one cycle per sec, 24/7 for two years on the first shot of grease. I kid you not. I stopped the test without any indication that we needed to re-lube, because we were tired of listening to it.
Less often, I've made ServoNuts with their 20x40Lead, and got 2.5m/sec out of them. (5,905 ipm!)