I think in terms of pens it tends to go: Cheap with "bad" lines to Expensive with "good" lines. In your case the opposite though.
"Bad" lines do what you describe. There isn't an even flow and pressing lightly gets less ink on the paper. Which can be good for using the pen as a sketch tool. Bad lines usually only come from cheap ballpoint or rollerball pens. You can get more expensive ones, or ones with different ink types such as gel, but they might flow better which is not what you want.
"Good" lines do the opposite. Usually a good line flows constantly, doesn't do what a bad line does, does not bleed, and depending on preference dries fast or permanent. They're considered good because they're the types of line needed for inking work (not sketching). Good lines usually come from pens with a fiber tip such as the prismacolors you tried. But there's huge range of tip types here (from sharpies to felt pens, etc). The good thing about fiber tip pens is they can get really small (below 0.5) and are great for detail work. The other type of pen that can get good lines are the ones with a thin steal nib (usually used for drafting, these are my favorite because they flow the best) and fountain pens and dip pens (which also flow super well but are slightly messier).
As you can see I don't think you'd want any of the "good" pens as they don't have the properties you want. I would like to perhaps make the suggestion that you sketch with the cheap pens and ink with the other ones if you want to find a use for them. Also Pilot came out with this amazing Frixion pen that erases by heat. I sketch with a blue or red one, ink on top, than get a hairdryer to erase the lines (the included eraser sucks, actual heat instead of friction is better). You might want to give them a try.
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 just suggested it because I've been wanting to do more ink sketching and sometimes I mess up completely or there's stray lines and I like the look of neat inked lines if I choose to finish the sketch so I bought those pens. Forgot to mention that if you erase all the lines you can make them come back by putting the drawing somewhere cold like a freezer (great for showing others your under drawing).
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).