Thanks for that detailed reply. You seem to sense what I'm after here, which I guess is the characteristic crappy ink flow of cheap pens! I'm realizing I don't want a smooth ink flow, because I'm sketching with pen, rather than inking, if that makes sense.
I actually like the fact that I can't erase these sketches, because the "mistakes" sometimes turn into something good, or at lest unexpected. They also sometimes ruin the picture, but that's OK. Somehow knowing I can't erase frees me up a bit, because I can't fret too much about trying to make it right. Once I make a line, I have to live with it. Even with the cheap pens, I can only sketch lightly a little bit, and then eventually I have to commit to a line and use it.
I'll be up-front and say that this heavily depends on what sort of work you want to do with the pen, and to a lesser extent the surface you're working on. Strictly from what I've worked with, those bottom-grade ballpoints are pretty much what you want if you're looking to achieve that multi-toned, sketchy effect. That said, however, it's still new ground for me. I've particularly been avoiding any pens labelled for smooth writing/flow like the plague as I've tried to refill my stock.
I really, really want to second Zebra pens in the event that you want bold, solid lines and don't want to use your Prismacolors for whatever reason, but I no longer seem to have any of my own—a shame, really, as I recall them being quality products. I do understand if you would need a ballpoint to fulfill the need, however, considering I myself have spongy drawing surfaces that drink up ink from free-flowing sources. I often use a Pilot G-Tec-C4 in the sketchbook in question...don't use it! The ink's super-thin and won't even hold when dry (why I don't use it anywhere else, and the paper takes poorly to an eraser anyway).
I think there are, but to be honest, I use a Pentel 1.0 mm ballpoint, which you can find just about anywhere. What I really like about it is that along with having various tones, it also draws very smoothly.
Doesn't the answer really depend a great deal on the person. I personally, do all my drawing using Zebra pens because I can draw very lightly with them. I don't bother going dark until I'm ready for a gel.